tools/testing/nvdimm: advertise a write cache for nfit_test

Commit 546eb031 "libnvdimm, pmem: Do not flush power-fail protected CPU caches"
fixed the write_cache detection to correctly show the lack of a write
cache based on the platform capabilities described in the ACPI NFIT. The
nfit_test unit tests expected a write cache to be present, so change the
nfit test namespaces to only advertise a persistence domain limited to
the memory controller. This allows the kernel to show a write_cache
attribute, and the test behaviour remains unchanged.

Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>