LU-16521 tests: racer should work without DNE

LU-16521 tests: racer should work without DNE

Don't assume there are multiple MDTs for the target directory
when running dir_migrate and dir_remote.  If only one MDT
(or none, if running on a non-Lustre directory for some reason),
just exit from these workloads instead of spewing errors.

Test-parameters: trivial testlist=sanityn
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I47cbe9de2c4f9afd79228b6a505dde023f2540e5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51267
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Alex Deiter <alex.deiter@gmail.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>