tools/testing/nvdimm: Make DSM failure code injection an override

In order to emulate the behavior of the NVDIMM_FAMILY_INTEL DSMs
nfit_test needs the ability to execute the DSM and then override the
return code.

Split the current return code injection from get_dimm() and apply at
after the function has executed to override the return status.

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