skip some checks if /sys/fs/selinux cannot be unmounted
    
    Some checks in the automated tests require an unmounted SELinux
    file-system. This way is used to pretend that SELinux is disabled.
    Sometimes the file-system cannot be unmounted because of
    
      umount: /sys/fs/selinux: target is busy.
    
    To work around this issue, umount with --lazy option will be used.
    If umount --lazy still complains, the problematic checks will be
    skipped, because it still make sense to run the other parts of the
    automated tests.