drm/tests: helpers: Create a helper to allocate an atomic state

As we gain more tests, boilerplate to allocate an atomic state and free
it starts to be there more and more as well.

In order to reduce the allocation boilerplate, we can create a helper
to create that atomic state, and call an action when the test is done.
This will also clean up the exit path.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Link: https://lore.kernel.org/r/20230728-kms-kunit-actions-rework-v3-6-952565ccccfe@kernel.org


Signed-off-by: Maxime Ripard <mripard@kernel.org>