Add missing override keyword:

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

src/test/app/HashRouter_test.cpp

@@ -284,7 +284,7 @@ class HashRouter_test : public beast::unit_test::suite
 public:
 
     void
-    run()
+    run() override
     {
         testNonExpiration();
         testExpiration();