drm/ttm/tests: Add TODO file

List improvements for the test suite with some notes.

Signed-off-by: Karolina Stolarek <karolina.stolarek@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3856fbbeaf40078d59b45971002b8f9d0d85f8f0.1718192625.git.karolina.stolarek@intel.com