Add missing override keyword:

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

src/test/core/ClosureCounter_test.cpp

@@ -313,7 +313,7 @@ class ClosureCounter_test : public beast::unit_test::suite
     }
 
 public:
-    void run()
+    void run() override
     {
         testConstruction();
         testArgs();