drm/tests: Remove slow tests

Both the drm_buddy and drm_mm tests have been converted from selftest to
kunit recently.

However, a significant portion of them are trying to exert some part of
their API over a huge number of iterations and with random variations of
their parameters.  They are thus more a way to discover new bugs than
actual unit tests.

This is fine in itself but leads to very slow runtime (up to 25s for
some drm_test_mm_reserve and drm_test_mm_insert on a Ryzen 7950x while
running the tests in qemu) which make them a poor fit for kunit.

Let's remove those tests from the drm_mm and drm_buddy test suites for
now, and if it's ever needed we can always create proper unit tests for
them later on.

This made the entire DRM tests execution time (as of v6.6-rc1) come from
65s to less than .5s on a Ryzen 7950x system when running under qemu,
and from 9 minutes to about 4s on a RaspberryPi4.

Acked-by: Daniel Vetter <daniel@ffwll.ch>
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Link: https://lore.kernel.org/r/20231025132428.723672-1-mripard@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>