cxl/test: Add mock test for set_timestamp

Support the command testing in a unit-test fashion.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Link: https://lore.kernel.org/r/20230423221231.6357-1-dave@stgolabs.net


Signed-off-by: Dan Williams <dan.j.williams@intel.com>