Add missing override keyword:

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

src/test/rpc/ValidatorRPC_test.cpp

@@ -399,7 +399,7 @@ public:
     }
 
     void
-    run()
+    run() override
     {
         testPrivileges();
         testStaticUNL();