drm/tests: Introduce a config option for the KUnit helpers

Driver-specific tests will need access to the helpers without pulling
every DRM framework test. Let's create an intermediate Kconfig options
for the helpers.

Suggested-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-2-4615a663a84a@cerno.tech


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