drm/vkms: Add KUnit test scaffolding

drm/vkms: Add KUnit test scaffolding

Add the required boilerplate to start creating KUnit test.

To run the tests:

    $ ./tools/testing/kunit/kunit.py run \
      --kunitconfig=drivers/gpu/drm/vkms/tests

Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Co-developed-by: Arthur Grillo <arthurgrillo@riseup.net>
Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Co-developed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250218101214.5790-4-jose.exposito89@gmail.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>