drm/tests: helpers: Move the helper header to include/drm

We'll need to use those helpers from drivers too, so let's move it to a
more visible location.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-1-4615a663a84a@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>