Merge bitcoin/bitcoin#27348: test: fix intermittent failure in ChainStateManager tests

f8abcb3 test: Fix intermittent failure in ChainStateManager tests (Martin Zumsande)

Pull request description:

  Before wiping a `ChainStateManager` and creating a new one, the `validationinterface` queue should be emptied to avoid the possibility of accessing deleted memory.
  This could lead to very rare CI failures reported in #26613 and #27320 (see [here](bitcoin/bitcoin#27320 (comment)) for a more detailed explanation).

  Fixes #27320

ACKs for top commit:
  jamesob:
    crACK bitcoin/bitcoin@f8abcb3
  MarcoFalke:
    lgtm ACK f8abcb3

Tree-SHA512: 953fcca46ffee039ae1c8c98eed5464ba7ee0007f54e18989caab3f645f5f45e64407a21f614984fd2843a3d56219f44603086b0c305a9a5cec64a8bf76c110a

Author: fanquake

src/test/validation_chainstatemanager_tests.cpp

@@ -367,10 +367,13 @@ struct SnapshotTestSetup : TestChain100Setup {
 
         BOOST_TEST_MESSAGE("Simulating node restart");
         {
-            LOCK(::cs_main);
             for (Chainstate* cs : chainman.GetAll()) {
+                LOCK(::cs_main);
                 cs->ForceFlushStateToDisk();
             }
+            // Process all callbacks referring to the old manager before wiping it.
+            SyncWithValidationInterfaceQueue();
+            LOCK(::cs_main);
             chainman.ResetChainstates();
             BOOST_CHECK_EQUAL(chainman.GetAll().size(), 0);
             const ChainstateManager::Options chainman_opts{