drm/xe/kunit: Add GuC Doorbells Manager tests

Add few tests to make sure that basic usage scenarios of the GuC
Doorbells Manager are implemented correctly.

Cc: Piotr Piórkowski <piotr.piorkowski@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://lore.kernel.org/r/20231218190629.502-11-michal.wajdeczko@intel.com
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>