test: Check btcsignals determinism in thread_safety test case

diff --git a/src/test/btcsignals_tests.cpp b/src/test/btcsignals_tests.cpp
index 7d7970bd..dd7fcb9b 100644
--- a/src/test/btcsignals_tests.cpp
+++ b/src/test/btcsignals_tests.cpp
@@ -111,17 +111,18 @@ BOOST_AUTO_TEST_CASE(any_of_combiner)
  * Connect sig0 to an incrementor function and loop in a thread.
  * Meanwhile, in another thread, inject and call new increment callbacks.
  * Both threads are constantly calling empty/connected.
- * Though the end-result is undefined due to a non-deterministic number of
- * total callbacks executed, this should all be completely threadsafe.
+ * The end-result must be deterministic for the atomic modified by conn0.
+ * Though, the end-result for the atomic modified by the extra connections is
+ * undefined due to a non-deterministic number of total callbacks executed.
+ * In any case, this should all be completely threadsafe.
  * Sanitizers should pick up any buggy data race behavior (if present).
  */
 BOOST_AUTO_TEST_CASE(thread_safety)
 {
     btcsignals::signal<void()> sig0;
-    std::atomic<uint32_t> val{0};
-    auto conn0 = sig0.connect([&val] {
-        val++;
-    });
+    std::atomic<uint32_t> val_det{0};
+    std::atomic<uint32_t> val_non_det{0};
+    auto conn0 = sig0.connect([&val_det] { val_det++; });
 
     std::thread incrementor([&conn0, &sig0] {
         for (int i = 0; i < 1000; i++) {
@@ -134,16 +135,17 @@ BOOST_AUTO_TEST_CASE(thread_safety)
         assert(conn0.connected());
     });
 
-    std::thread extra_increment_injector([&conn0, &sig0, &val] {
+    std::thread extra_increment_injector([&conn0, &sig0, &val_non_det] {
         static constexpr size_t num_extra_conns{1000};
         std::vector<btcsignals::scoped_connection> extra_conns;
         extra_conns.reserve(num_extra_conns);
         for (size_t i = 0; i < num_extra_conns; i++) {
             BOOST_CHECK(!sig0.empty());
             BOOST_CHECK(conn0.connected());
-            extra_conns.emplace_back(sig0.connect([&val] {
-                val++;
-            }));
+            btcsignals::scoped_connection extra{sig0.connect([&val_non_det] { val_non_det++; })};
+            if (i % 2 == 0) {
+                extra_conns.emplace_back(std::move(extra));
+            }
             sig0();
         }
     });
@@ -152,10 +154,16 @@ BOOST_AUTO_TEST_CASE(thread_safety)
     conn0.disconnect();
     BOOST_CHECK(sig0.empty());
 
-    // sig will have been called 2000 times, and at least 1000 of those will
-    // have been executing multiple incrementing callbacks. So while val is
-    // probably MUCH bigger, it's guaranteed to be at least 3000.
-    BOOST_CHECK_GE(val.load(), 3000);
+    // sig0 will have been called 2000 times, and only the first connection did
+    // increment val_det, so it must be 2000.
+    BOOST_CHECK_EQUAL(val_det.load(), 2000);
+    // The number of connections that increment val_non_det is growing from 1
+    // to 500, where 500 are disconnected immediately again after the step.
+    // Before the end of each step the connections are called at least once.
+    // However, it is unknown how often the connections have been called
+    // exactly. The 500th Triangular Number gives a lower estimate.
+    // T_n=n(n+1)/2
+    BOOST_CHECK_GE(val_non_det.load(), 500 * 501 / 2);
 }
 
 /* Test that connection and disconnection works from within signal