LU-13709 tests: test lfs mkdir -c without -i

LU-13709 tests: test lfs mkdir -c without -i

Almost every test with lfs mkdir -c in the test suite also
uses option -i, so lfs mkdir -c (same as -i -1, where lustre
chooses the MDTs) is poorly tested.  Add a test for that
case, sanity test_300s.

Lustre-change: https://review.whamcloud.com/39457
Lustre-commit: 2c89cc6c25549cb6748c7c9f5a209c7e38387eb4

Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Change-Id: Iede537d52cf445c9c9a6353338670e55a11364da
Reviewed-on: https://review.whamcloud.com/40801
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Gian-Carlo DeFazio <defazio1@llnl.gov>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>