drm/xe/tests: Move shrink test out of xe_bo

drm/xe/tests: Move shrink test out of xe_bo

The subtest typically has an execution time long enough to motivate a
separate test so that it can be easily excluded if needed.

v2: reword commit message(Thomas)

Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241218141447.2528530-1-nirmoy.das@intel.com
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>