drm/vkms: Add tests for CTM handling

drm/vkms: Add tests for CTM handling

A whole slew of tests for CTM handling that greatly helped in
debugging the CTM code. The extent of tests might seem a bit
silly but they're fast and might someday help save someone
else's day when debugging this.

Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patch.msgid.link/20251115000237.3561250-23-alex.hung@amd.com