Add missing override keyword:

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

src/test/app/SetTrust_test.cpp

@@ -204,7 +204,7 @@ public:
 
     }
 
-    void run()
+    void run() override
     {
         testFreeTrustlines(true, false);
         testFreeTrustlines(false, true);