Add missing override keyword:

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

src/test/basics/StringUtilities_test.cpp

@@ -86,7 +86,7 @@ public:
         BEAST_EXPECT(result == "hello");
     }
 
-    void run ()
+    void run () override
     {
         testParseUrl ();
         testUnHex ();