LU-6006 tests: add sleep 1 after command in background

LU-6006 tests: add sleep 1 after command in background

otherwise subsequent command may race with the one in
background and fail:
 mkdir a & touch a/b

Test-Parameters: trivial env=ONLY="22-23",ONLY_REPEAT=50 testlist=replay-dual
Test-Parameters: env=ONLY="22-23",ONLY_REPEAT=50 fstype=zfs testlist=replay-dual
Test-Parameters: env=ONLY="22-23",ONLY_REPEAT=50 mdscount=2 mdtcount=4 testlist=replay-dual
Test-Parameters: env=ONLY="22-23",ONLY_REPEAT=50 fstype=zfs mdscount=2 mdtcount=4 testlist=replay-dual

Lustre-change: https://review.whamcloud.com/37343
Lustre-commit: 2d57999401072a034650d00a37fb59ef9b3f53d0

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I0574d315e596cd899f7c4ea20c70b4c3da99b9b4
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/45916
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>