g_main_loop_quit() only quits mainloops that are currently running --
not ones that may run in the future.  The way the gdbus-threading tests
are written can possibly result in a call to g_main_loop_quit() before
g_main_loop_run() has started.

The mainloops aren't actually used for anything other than signalling
the completion of the threads, so just use g_thread_join() for that.

https://bugzilla.gnome.org/show_bug.cgi?id=666129