LU-14909 test: mkdir_on_mdt0 to mkdir on MDT0

LU-14909 test: mkdir_on_mdt0 to mkdir on MDT0

Many sub tests in recovery-small.sh and replay-single.sh need to mkdir
on MDT0, use mkdir_on_mdt0() to create such directories.

Lustre-change: https://review.whamcloud.com/44544
Lustre-commit: 96fd8f03c57d778fd40055b58f54b7310b704adc

Test-Parameters: trivial mdscount=2 mdtcount=4 testlist=replay-single.sh
Test-Parameters: mdscount=2 mdtcount=4 testlist=recovery-small.sh
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Id4a44da062350ea284f51c8c821302aebbfe9dee
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/49946
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>