selinux.security_getenforce() triggers an exception when running tests
on systems without SELinux. In order to skip tests which need SELinux in
enforcing mode, test selinux.is_selinux_enabled() too, like commit
945bc8853b ("sandbox: make test not fail on systems without SELinux").
Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>