drm/xe/tests: Grab a memory access reference around the migrate sanity test

It appears we don't hold a memory access reference for the accesses in
this test, which may results in printed warnings and possibly the GT
not woken up for the memory accesses.

Add a memory access reference around the test.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>