drm/ttm: Introduce KUnit test

Add the initial version of unit tests for ttm_device struct, together
with helper functions.

Signed-off-by: Karolina Stolarek <karolina.stolarek@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3d1cc45c8a0cf536b92a850e0025f6c555de0169.1691487006.git.karolina.stolarek@intel.com


Signed-off-by: Christian König <christian.koenig@amd.com>