LU-18605 tests: give time to start in sanityn/31s

LU-18605 tests: give time to start in sanityn/31s

give tail command some time to start otherwise it can race with
mv command few lines ago and if mv wins then the test fail with:
sanityn.sh: line 1218: cd: /mnt/lustre/d31s.sanityn: No such file
tail: cannot open 'f31s.sanityn' for reading: No such file
tail: no files remaining
 sanityn test_31s: @@@@@@ FAIL: fail to stop 21777

Test-Parameters: trivial
Fixes: 32582842ca ("LU-17583 llite: getattr/open should not revalidate dentry")
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ibff49cf12bebc4799eb0eefe2a4e45bfa459f660
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57606
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>