Add missing override keyword:

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

src/test/rpc/TransactionHistory_test.cpp

@@ -142,7 +142,7 @@ class TransactionHistory_test : public beast::unit_test::suite
     }
 
 public:
-    void run ()
+    void run () override
     {
         testBadInput();
         testRequest();