LU-13580 tests: fix retrieval of SELinux context

Use 'stat' command instead of 'ls -lZ' to retrieve SELinux security
context, to make it more portable.

Test-Parameters: trivial clientselinux testlist=sanity-selinux mdtcount=2 clientcount=2
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I61bc0efb1e8ae0427d05827e2933eb0b848fb442
Reviewed-on: https://review.whamcloud.com/38648
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>