Set XDG_DATA_DIRS to make sure we don't use /usr/share from the
appmonitor test. We will soon throw a warning if we find defaults.list,
so make sure we don't open ourselves up to that if there is one on the
system.
https://bugzilla.gnome.org/show_bug.cgi?id=728040
The test reveals that there's something fishy with this monitor.
One has to call g_app_info_get_all() for it to start working,
and then it only works once.
