drm/tests: helpers: Create a helper to allocate a locking ctx

As we get more and more tests, the locking context initialisation
creates more and more boilerplate, both at creation and destruction.

Let's create a helper that will allocate, initialise a context, and
register kunit actions to clean up once the test is done.

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-v3-5-952565ccccfe@kernel.org


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