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.