menu threaded test: run the mainloop after export

GDBusConnection now dispatches GDestroyNotify calls back to the
mainloop.  Adding an idle to the mainloop is O(n) in the number of idles
already there.  We therefore need to periodically empty the mainloop to
avoid quadratic behaviour with a very large 'n'.

gio/tests/gmenumodel.c

@@ -754,6 +754,7 @@ do_export (gpointer data)
       id = g_dbus_connection_export_menu_model (bus, path, menu, &error);
       g_assert_no_error (error);
       g_dbus_connection_unexport_menu_model (bus, id);
+      while (g_main_context_iteration (NULL, FALSE));
     }
 
   g_free (path);