LU-13417 test: generate uneven MDTs early for sanity 413

LU-13417 test: generate uneven MDTs early for sanity 413

Fill MDT early to generate uneven MDTs for sanity test_413, and
add test_413z to unlink these directories.

Test-Parameters: trivial
Test-Parameters: testgroup=review-dne-part-1
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I84e3670bb40c3666488139d6a272f29188b0dfae
Reviewed-on: https://review.whamcloud.com/44384
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>