Add missing override keyword:

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

src/test/protocol/Issue_test.cpp

@@ -456,7 +456,7 @@ public:
 
     //--------------------------------------------------------------------------
 
-    void run()
+    void run() override
     {
         testcase ("Currency");
         testUnsigned <Currency> ();