LU-17786 tests: use $TSTUSR instead of hard coding quota_usr

LU-17786 tests: use $TSTUSR instead of hard coding quota_usr

The bash function check_system_is_clean() hard codes the user.
For many external system due to security we can't create special
users so use $TSTUSR instead that can already exits for us.

Change-Id: I80d522f04bc813cd6d5aef000eeeb34d6ec81ebd
Fixes: 7e1fb1a296e ("LU-17179 tests: check the system is clean")
Test-Parameters: trivial testlist=sanity-quota
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54940
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sergey Cheremencev <scherementsev@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>