testsuite: Destroy surfaces

If we don't destroy the surface, it leaks.

GDK backends keep an extra reference on the
surface for the external resources associated
with it, and only drop it in destroy().

testsuite/gtk/shortcuts.c

@@ -356,6 +356,7 @@ test_trigger_trigger (void)
       gdk_event_unref (event);
     }
 
+  gdk_surface_destroy (surface);
   g_object_unref (surface);
 
   g_object_unref (trigger[0]);