Add missing override keyword:

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

src/test/basics/contract_test.cpp

@@ -26,7 +26,7 @@ namespace ripple {
 class contract_test : public beast::unit_test::suite
 {
 public:
-    void run ()
+    void run () override
     {
         try
         {