Documentation: gpu: vc4: Add KUnit Tests Section

Now that we have VC4-specific tests in place, let's document them
properly.

Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-20-4615a663a84a@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>