tests: Work around a crash in the new gtk4 a11y stack
tests: Work around a crash in the new gtk4 a11y stack

gtk4 calls get_dbus_object_path() on the default app and glib leaves
invalid GApplication instances as default in various places. It's hard
to track where a new default instances gets set, so just disable a11y
in the test suite for now.

GNOME/pygobject,GNOME/pygobject,GNOME/pygobject