itest: assert sweep outpoint mined after RBF

diff --git a/itest/lnd_sweep_test.go b/itest/lnd_sweep_test.go
index 3874786..2b9c32e 100644
--- a/itest/lnd_sweep_test.go
+++ b/itest/lnd_sweep_test.go
@@ -689,8 +689,20 @@ func testSweepCPFPAnchorIncomingTimeout(ht *lntest.HarnessTest) {
 	// contractcourt will offer the HTLC to his sweeper. We are not testing
 	// the HTLC sweeping behaviors so we just perform a simple check and
 	// exit the test.
-	ht.AssertNumPendingSweeps(bob, 1)
-	ht.MineBlocksAndAssertNumTxes(1, 1)
+	htlcSweep := ht.AssertNumPendingSweeps(bob, 1)[0]
+	htlcSweepOutpointHash, err := chainhash.NewHashFromStr(
+		htlcSweep.Outpoint.TxidStr,
+	)
+	require.NoError(ht, err)
+	htlcSweepOutpoint := wire.OutPoint{
+		Hash:  *htlcSweepOutpointHash,
+		Index: htlcSweep.Outpoint.OutputIndex,
+	}
+
+	// The final sweep may be RBFed between the mempool check and block
+	// generation, so assert that the mined tx spends the pending sweep's
+	// outpoint instead of asserting the txid observed before mining.
+	ht.MineBlockAndAssertOutpointSpent(1, htlcSweepOutpoint)
 
 	// Finally, clean the mempool for the next test.
 	ht.CleanShutDown()
diff --git a/lntest/harness_miner.go b/lntest/harness_miner.go
index bc24b5e..0445f07 100644
--- a/lntest/harness_miner.go
+++ b/lntest/harness_miner.go
@@ -121,6 +121,66 @@ func (h *HarnessTest) MineBlocksAndAssertNumTxes(num uint32,
 	return blocks
 }
 
+// MineBlockAndAssertOutpointSpent mines a block and asserts the given outpoint
+// was spent in it. Unlike MineBlocksAndAssertNumTxes, it does not require the
+// txids seen in the mempool before mining to be the txids that confirm. This is
+// useful for RBF-sensitive flows where a transaction may be replaced between
+// the mempool check and block generation.
+func (h *HarnessTest) MineBlockAndAssertOutpointSpent(numTxs int,
+	outpoint wire.OutPoint) *wire.MsgBlock {
+
+	// Update the harness's current height.
+	defer h.updateCurrentHeight()
+
+	// If we expect transactions to be included in the blocks we'll mine,
+	// wait until they are seen in the miner's mempool.
+	h.AssertNumTxsInMempool(numTxs)
+
+	// Mine a block.
+	block := h.miner.MineBlocks(1)[0]
+
+	// Assert that the expected number of non-coinbase transactions were
+	// included in the block.
+	require.Len(h, block.Transactions, numTxs+1)
+
+	// Assert that the expected outpoint was spent in the block.
+	h.assertOutpointSpentInBlock(block, outpoint)
+
+	// Finally, make sure all the active nodes are synced.
+	h.AssertActiveNodesSyncedTo(block.BlockHash())
+
+	return block
+}
+
+// assertOutpointSpentInBlock asserts that the given outpoint is spent by a
+// transaction in the passed block.
+func (h *HarnessTest) assertOutpointSpentInBlock(block *wire.MsgBlock,
+	outpoint wire.OutPoint) {
+
+	var txids []chainhash.Hash
+	var prevouts []wire.OutPoint
+
+	for _, tx := range block.Transactions {
+		if blockchain.IsCoinBaseTx(tx) {
+			continue
+		}
+
+		txids = append(txids, tx.TxHash())
+
+		for _, txIn := range tx.TxIn {
+			prevouts = append(prevouts, txIn.PreviousOutPoint)
+
+			if txIn.PreviousOutPoint == outpoint {
+				return
+			}
+		}
+	}
+
+	require.Failf(h, "outpoint was not spent in block",
+		"outpoint:%v, block:%v, txids:%v, prevouts:%v",
+		outpoint, block.BlockHash(), txids, prevouts)
+}
+
 // ConnectMiner connects the miner with the chain backend in the network.
 func (h *HarnessTest) ConnectMiner() {
 	err := h.manager.chainBackend.ConnectMiner()