tools/testing/nvdimm: test get_config_size DSM failures

Add an nfit_test specific attribute for gating whether a get_config_size
DSM, or any DSM for that matter, succeeds or fails.  The get_config_size
DSM is initial motivation since that is the first command libnvdimm core
issues to determine the state of the namespace label area.

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