tools/testing/nvdimm: Prepare nfit_ctl_test() for ND_CMD_CALL emulation

In preparation for adding a mocked implementation of the
firmware-activate bus-info command, rework nfit_ctl_test() to operate on
a local command payload wrapped in a 'struct nd_cmd_pkg'.

Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>