app/tests: Run main loop until idle instead of for a given time

Use gimp_test_run_mainloop_until_idle() instead of
gimp_test_run_temp_mainloop () since we then don't have to wait for a
timeout, we only wait the minimal amount of time.

app/tests/test-session-management.c

@@ -85,7 +85,7 @@ int main(int argc, char **argv)
    * to let things stabilize. This includes parsing sessionrc and
    * dockrc
    */
-  gimp_test_run_temp_mainloop (4000);
+  gimp_test_run_mainloop_until_idle ();
 
   /* Exit. This includes writing sessionrc and dockrc*/
   gimp_exit (gimp, TRUE);