Add a stress-test for GDBusProxy in threads with no default main context

Add a stress-test for GDBusProxy in threads with no default main context

Destroying a GDBusProxy in a thread used to race with NameOwnerChanged
being delivered to the main context's thread (GNOME #651133).

Also, g_dbus_proxy_call_sync in a thread would race with NameOwnerChanged
being delivered to the main context's thread and rewriting the name_owner
(GNOME #656039).

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=656039
Bug-NB: NB#259760
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Signed-off-by: David Zeuthen <davidz@redhat.com>