Add missing override keyword:

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

src/test/app/AmendmentTable_test.cpp

@@ -757,7 +757,7 @@ public:
         BEAST_EXPECT(table->hasUnsupportedEnabled());
     }
 
-    void run ()
+    void run () override
     {
         testConstruct();
         testGet ();