drm/tests: Add a test for DRM managed actions

DRM-managed actions are supposed to be ran whenever the device is
released. Let's introduce a basic unit test to make sure it happens.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-12-4615a663a84a@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>