LU-15626 tests: Fix "error" reported by shellcheck for sanity.sh

LU-15626 tests: Fix "error" reported by shellcheck for sanity.sh

This patch fixes "error" issues reported by shellcheck
for file lustre/tests/sanity.sh

Test-Parameters: trivial testlist=sanity
Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I4223aff8bf740ba5710f58fae697768f3f704591
Reviewed-on: https://review.whamcloud.com/46741
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>