Add missing override keyword:

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

src/test/protocol/STAmount_test.cpp

@@ -606,7 +606,7 @@ public:
 
     //--------------------------------------------------------------------------
 
-    void run ()
+    void run () override
     {
         testSetValue ();
         testNativeCurrency ();