drm/todo: Add entry to clean up former seltests suites

Most of those suites are undocumented and aren't really clear about what
they are testing. Let's add a TODO entry as a future task to get started
into KUnit and DRM.

Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: https://lore.kernel.org/r/20231025132428.723672-2-mripard@kernel.org


Signed-off-by: Maxime Ripard <mripard@kernel.org>