Skip unreliable test_threaded_singleton() by default
Skip unreliable test_threaded_singleton() by default

This test aims to reproduce a race condition between last-unref of the
global singleton GDBusConnection and g_bus_get_sync(). However, test
setup intermittently times out with:

    # GLib-GIO-DEBUG: run 0: refcount is 2, sleeping
    Bail out! GLib-GIO-FATAL-ERROR: connection had too many refs

The current theory upstream is that this might be a reference leak in
test_delivery_in_thread().

Demote test_threaded_singleton() to be run as one of the "flaky"
autopkgtests, but not at build time or in the part of the autopkgtest
run that gates progress into testing.

Bug: https://gitlab.gnome.org/GNOME/glib/issues/1515
Forwarded: no

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib