Add missing override keyword:

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

src/test/app/SetRegularKey_test.cpp

@@ -92,7 +92,7 @@ public:
         env(jv, ter(temINVALID_FLAG));
     }
 
-    void run()
+    void run() override
     {
         testDisableMasterKey();
         testPasswordSpent();