test: add basic tests for ChainstateManager

Feedback incorporated from Russell Yanofsky.

Co-authored-by: MarcoFalke <falke.marco@gmail.com>

src/qt/test/apptests.cpp

@@ -82,6 +82,7 @@ void AppTests::appTests()
     // Reset global state to avoid interfering with later tests.
     AbortShutdown();
     UnloadBlockIndex();
+    g_chainman.Reset();
 }
 
 //! Entry point for BitcoinGUI tests.