Add missing override keyword:

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

src/test/peerfinder/PeerFinder_test.cpp

@@ -149,7 +149,7 @@ public:
         BEAST_EXPECT(n <= (seconds+59)/60);
     }
 
-    void run ()
+    void run () override
     {
         test_backoff1();
         test_backoff2();