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: [[ https://bugs.freebsd.org/191191 | 191191 ]]
Sponsored by: EMC / Isilon Storage Division