test: move MockMempoolMinFee to util/txmempool

diff --git a/src/test/txpackage_tests.cpp b/src/test/txpackage_tests.cpp
index 1f167586..ded13c4c 100644
--- a/src/test/txpackage_tests.cpp
+++ b/src/test/txpackage_tests.cpp
@@ -357,7 +357,7 @@ BOOST_AUTO_TEST_CASE(package_submission_tests)
 {
     // Mine blocks to mature coinbases.
     mineBlocks(3);
-    MockMempoolMinFee(CFeeRate(5000));
+    MockMempoolMinFee(CFeeRate(5000), *m_node.mempool);
     LOCK(cs_main);
     unsigned int expected_pool_size = m_node.mempool->size();
     CKey parent_key = GenerateRandomKey();
@@ -634,7 +634,7 @@ BOOST_AUTO_TEST_CASE(package_witness_swap_tests)
 {
     // Mine blocks to mature coinbases.
     mineBlocks(5);
-    MockMempoolMinFee(CFeeRate(5000));
+    MockMempoolMinFee(CFeeRate(5000), *m_node.mempool);
     LOCK(cs_main);
 
     // Transactions with a same-txid-different-witness transaction in the mempool should be ignored,
@@ -867,7 +867,7 @@ BOOST_AUTO_TEST_CASE(package_witness_swap_tests)
 BOOST_AUTO_TEST_CASE(package_cpfp_tests)
 {
     mineBlocks(5);
-    MockMempoolMinFee(CFeeRate(5000));
+    MockMempoolMinFee(CFeeRate(5000), *m_node.mempool);
     LOCK(::cs_main);
     size_t expected_pool_size = m_node.mempool->size();
     CKey child_key = GenerateRandomKey();
diff --git a/src/test/util/setup_common.cpp b/src/test/util/setup_common.cpp
index 8ce0a0db..baedcf0b 100644
--- a/src/test/util/setup_common.cpp
+++ b/src/test/util/setup_common.cpp
@@ -571,40 +571,6 @@ std::vector<CTransactionRef> TestChain100Setup::PopulateMempool(FastRandomContex
     return mempool_transactions;
 }
 
-void TestChain100Setup::MockMempoolMinFee(const CFeeRate& target_feerate)
-{
-    LOCK2(cs_main, m_node.mempool->cs);
-    // Transactions in the mempool will affect the new minimum feerate.
-    assert(m_node.mempool->size() == 0);
-    // The target feerate cannot be too low...
-    // ...otherwise the transaction's feerate will need to be negative.
-    assert(target_feerate > m_node.mempool->m_opts.incremental_relay_feerate);
-    // ...otherwise this is not meaningful. The feerate policy uses the maximum of both feerates.
-    assert(target_feerate > m_node.mempool->m_opts.min_relay_feerate);
-
-    // Manually create an invalid transaction. Manually set the fee in the CTxMemPoolEntry to
-    // achieve the exact target feerate.
-    CMutableTransaction mtx = CMutableTransaction();
-    mtx.vin.emplace_back(COutPoint{Txid::FromUint256(m_rng.rand256()), 0});
-    mtx.vout.emplace_back(1 * COIN, GetScriptForDestination(WitnessV0ScriptHash(CScript() << OP_TRUE)));
-    // Set a large size so that the fee evaluated at target_feerate (which is usually in sats/kvB) is an integer.
-    // Otherwise, GetMinFee() may end up slightly different from target_feerate.
-    BulkTransaction(mtx, 4000);
-    const auto tx{MakeTransactionRef(mtx)};
-    LockPoints lp;
-    // The new mempool min feerate is equal to the removed package's feerate + incremental feerate.
-    const auto tx_fee = target_feerate.GetFee(GetVirtualTransactionSize(*tx)) -
-        m_node.mempool->m_opts.incremental_relay_feerate.GetFee(GetVirtualTransactionSize(*tx));
-    {
-        auto changeset = m_node.mempool->GetChangeSet();
-        changeset->StageAddition(tx, /*fee=*/tx_fee,
-                /*time=*/0, /*entry_height=*/1, /*entry_sequence=*/0,
-                /*spends_coinbase=*/true, /*sigops_cost=*/1, lp);
-        changeset->Apply();
-    }
-    m_node.mempool->TrimToSize(0);
-    assert(m_node.mempool->GetMinFee() == target_feerate);
-}
 /**
  * @returns a real block (0000000000013b8ab2cd513b0261a14096412195a72a0c4827d229dcc7e0f7af)
  *      with 9 txs.
diff --git a/src/test/util/setup_common.h b/src/test/util/setup_common.h
index 150f5065..fdb0951e 100644
--- a/src/test/util/setup_common.h
+++ b/src/test/util/setup_common.h
@@ -238,17 +238,6 @@ struct TestChain100Setup : public TestingSetup {
      */
     std::vector<CTransactionRef> PopulateMempool(FastRandomContext& det_rand, size_t num_transactions, bool submit);
 
-    /** Mock the mempool minimum feerate by adding a transaction and calling TrimToSize(0),
-     * simulating the mempool "reaching capacity" and evicting by descendant feerate.  Note that
-     * this clears the mempool, and the new minimum feerate will depend on the maximum feerate of
-     * transactions removed, so this must be called while the mempool is empty.
-     *
-     * @param target_feerate    The new mempool minimum feerate after this function returns.
-     *                          Must be above max(incremental feerate, min relay feerate),
-     *                          or 1sat/vB with default settings.
-     */
-    void MockMempoolMinFee(const CFeeRate& target_feerate);
-
     std::vector<CTransactionRef> m_coinbase_txns; // For convenience, coinbase transactions
     CKey coinbaseKey; // private/public key needed to spend coinbase transactions
 };
diff --git a/src/test/util/txmempool.cpp b/src/test/util/txmempool.cpp
index 5febb679..441cdbb2 100644
--- a/src/test/util/txmempool.cpp
+++ b/src/test/util/txmempool.cpp
@@ -10,6 +10,7 @@
 #include <policy/rbf.h>
 #include <policy/truc_policy.h>
 #include <txmempool.h>
+#include <test/util/transaction_utils.h>
 #include <util/check.h>
 #include <util/time.h>
 #include <util/translation.h>
@@ -218,3 +219,38 @@ void AddToMempool(CTxMemPool& tx_pool, const CTxMemPoolEntry& entry)
             entry.GetSpendsCoinbase(), entry.GetSigOpCost(), entry.GetLockPoints());
     changeset->Apply();
 }
+
+void MockMempoolMinFee(const CFeeRate& target_feerate, CTxMemPool& mempool)
+{
+    LOCK2(cs_main, mempool.cs);
+    // Transactions in the mempool will affect the new minimum feerate.
+    assert(mempool.size() == 0);
+    // The target feerate cannot be too low...
+    // ...otherwise the transaction's feerate will need to be negative.
+    assert(target_feerate > mempool.m_opts.incremental_relay_feerate);
+    // ...otherwise this is not meaningful. The feerate policy uses the maximum of both feerates.
+    assert(target_feerate > mempool.m_opts.min_relay_feerate);
+
+    // Manually create an invalid transaction. Manually set the fee in the CTxMemPoolEntry to
+    // achieve the exact target feerate.
+    CMutableTransaction mtx{};
+    mtx.vin.emplace_back(COutPoint{Txid::FromUint256(uint256{123}), 0});
+    mtx.vout.emplace_back(1 * COIN, GetScriptForDestination(WitnessV0ScriptHash(CScript() << OP_TRUE)));
+    // Set a large size so that the fee evaluated at target_feerate (which is usually in sats/kvB) is an integer.
+    // Otherwise, GetMinFee() may end up slightly different from target_feerate.
+    BulkTransaction(mtx, 4000);
+    const auto tx{MakeTransactionRef(mtx)};
+    LockPoints lp;
+    // The new mempool min feerate is equal to the removed package's feerate + incremental feerate.
+    const auto tx_fee = target_feerate.GetFee(GetVirtualTransactionSize(*tx)) -
+        mempool.m_opts.incremental_relay_feerate.GetFee(GetVirtualTransactionSize(*tx));
+    {
+        auto changeset = mempool.GetChangeSet();
+        changeset->StageAddition(tx, /*fee=*/tx_fee,
+                /*time=*/0, /*entry_height=*/1, /*entry_sequence=*/0,
+                /*spends_coinbase=*/true, /*sigops_cost=*/1, lp);
+        changeset->Apply();
+    }
+    mempool.TrimToSize(0);
+    assert(mempool.GetMinFee() == target_feerate);
+}
diff --git a/src/test/util/txmempool.h b/src/test/util/txmempool.h
index 36caad2a..731b46a3 100644
--- a/src/test/util/txmempool.h
+++ b/src/test/util/txmempool.h
@@ -67,4 +67,17 @@ void CheckMempoolTRUCInvariants(const CTxMemPool& tx_pool);
  *  and applying it. */
 void AddToMempool(CTxMemPool& tx_pool, const CTxMemPoolEntry& entry);
 
+/** Mock the mempool minimum feerate by adding a transaction and calling TrimToSize(0),
+ * simulating the mempool "reaching capacity" and evicting by descendant feerate.  Note that
+ * this clears the mempool, and the new minimum feerate will depend on the maximum feerate of
+ * transactions removed, so this must be called while the mempool is empty.
+ *
+ * @param target_feerate    The new mempool minimum feerate after this function returns.
+ *                          Must be above max(incremental feerate, min relay feerate),
+ *                          or 1sat/vB with default settings.
+ * @param mempool           The mempool to mock the minimum feerate for. Must be empty
+ *                          when called.
+ */
+void MockMempoolMinFee(const CFeeRate& target_feerate, CTxMemPool& mempool);
+
 #endif // BITCOIN_TEST_UTIL_TXMEMPOOL_H