Stabilize bug676

The test should now be stabler as it waits for a monitor interval instead
of a set amount of seconds.

maxscale-system-test/bug676.cpp

@@ -26,7 +26,7 @@ int main(int argc, char *argv[])
     test.stop_timeout();
 
     test.tprintf("Waiting until the monitor picks a new master");
-    sleep(5);
+    test.maxscales->wait_for_monitor();
 
     test.set_timeout(30);