Add missing override keyword:

* Enable the `suggest-override` warning for gcc
* Fix all functions that were flagged by that warning

src/test/core/Config_test.cpp

@@ -852,7 +852,7 @@ trustthesevalidators.gov
         }
     }
 
-    void run ()
+    void run () override
     {
         testLegacy ();
         testDbPath ();