drm/xe/kunit: Add KUnit tests for GuC Buffer Cache

Add tests to make sure that recently added GuC Buffer Cache
component is working as expected.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250114192140.1039-1-michal.wajdeczko@intel.com