drm/ttm/tests: Add tests for ttm_resource and ttm_sys_man

Test initialization of ttm_resource using different memory domains.
Add tests for a system memory manager and functions that can be
tested without a fully-featured resource manager. Update
ttm_bo_kunit_init() to initialize BO's kref and a genuine GEM drm
object. Export ttm_resource_alloc for test purposes only.

Signed-off-by: Karolina Stolarek <karolina.stolarek@intel.com>
Tested-by: Amaranath Somalapuram <Amaranath.Somalapuram@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/68b8b293b6bf5f1170d49a1a089ccce9172e2855.1701257386.git.karolina.stolarek@intel.com
Signed-off-by: Christian König <christian.koenig@amd.com>