drm/sched/tests: Add unit test for cancel_job()

drm/sched/tests: Add unit test for cancel_job()

The scheduler unit tests now provide a new callback, cancel_job(). This
callback gets used by drm_sched_fini() for all still pending jobs to
cancel them.

Implement a new unit test to test this.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://lore.kernel.org/r/20250710125412.128476-6-phasta@kernel.org