drm/tests: client: Mention that we can't use MODULE_ macros

That file is included directly, so we can't use any MODULE macro. Let's
leave a comment to avoid any future mistake.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Acked-in-principle-or-something-like-that-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20220728-rpi-analog-tv-properties-v10-1-256dad125326@cerno.tech


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