Add missing override keyword:

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

src/test/basics/Buffer_test.cpp

@@ -34,7 +34,7 @@ struct Buffer_test : beast::unit_test::suite
         return b.data() != nullptr;
     }
 
-    void run()
+    void run() override
     {
         std::uint8_t const data[] =
         {