Add missing override keyword:

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

src/test/rpc/Status_test.cpp

@@ -83,7 +83,7 @@ private:
     }
 
 public:
-    void run()
+    void run() override
     {
         test_OK ();
         test_error ();
@@ -200,7 +200,7 @@ private:
     }
 
 public:
-    void run()
+    void run() override
     {
         test_OK ();
         test_error ();