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

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

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

Test-Parameters: trivial testlist=sanity-dom
Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I185622dcb30abbc43a2118cbfaa9c643f8ce52bc
Reviewed-on: https://review.whamcloud.com/46796
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>