Comment 2 for bug 741186

Yeah, this is because of the way the system-tools-backends and liboobs are designed: oobs_object_authenticate() is a synchronous call. I don't think this will be fixed since the gnome-system-tools are deprecated in favor of GNOME3's control center - unless you redesign the whole API *or* introduce a hack so that users-admin first authenticates via a direct call to PolicyKit (action org.freedesktop.systemtoolsbackends.set) from around src/user-settings.c:1050. But that would only work for that precise action and would be a ugly hack. Not sure it's worth it...

Easier would be to setup your test machine so that action org.freedesktop.systemtoolsbackends.set is allowed without authentication.