MFC r275170:

  Convert sbin/mdconfig/tests from prove format tests to ATF format tests

  As a side effect...
  1. The tests now checks for the root user before continuing with kyua, which is
     more visible than the test being skipped with the TAP protocol
  2. The tests work with devices that aren't /dev/md0 by caching the device
     attached during the test to a file, and later use the cached information to
     detach the device in the cleanup routine
  3. The tests no longer require perl to run

  PR: 191191
  Sponsored by: EMC / Isilon Storage Division