test: Fix intermittent failure in ChainStateManager tests

Before wiping the ChainStateManager, the validationinterface
queue must be drained to avoid accessing deleted memory.

Author: mzumsande

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{