Add missing override keyword:

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

src/test/nodestore/Backend_test.cpp

@@ -100,7 +100,7 @@ public:
 
     //--------------------------------------------------------------------------
 
-    void run ()
+    void run () override
     {
         std::uint64_t const seedValue = 50;