Add missing override keyword:

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

src/test/shamap/SHAMap_test.cpp

@@ -117,7 +117,7 @@ public:
         return vuc;
     }
 
-    void run ()
+    void run () override
     {
         run (true,  SHAMap::version{1});
         run (false, SHAMap::version{1});