Add missing override keyword:

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

src/test/protocol/digest_test.cpp

@@ -148,7 +148,7 @@ public:
         pass ();
     }
 
-    void run ()
+    void run () override
     {
         testSHA512 ();
         testSHA256 ();