Instead of testing headless start using the dummy backend, do so with
the real native backend, and use the drm-mock library instead to emulate
monitors being disconnected at startup.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2821>
This new filter allows test cases to manipulate what the kernel reports,
e.g. mark connected connectors as disconnected to emulate monitors
connecting and disconnecting.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2821>