tools/testing/nvdimm: unit test for acpi_nvdimm_notify()

Trigger an nmemX/nfit/flags attribute to fire an event whenever a
smart-threshold DSM is received.

Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>