Add missing override keyword:

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

src/test/app/SHAMapStore_test.cpp

@@ -623,7 +623,7 @@ public:
         lastRotated = ledgerSeq - 1;
     }
 
-    void run()
+    void run() override
     {
         testClear();
         testAutomatic();