ipatests: fix TestUserPermissions::test_selinux_user_optimized
This test requires SELinux and fails if selinux is disabled
(because it's calling semanage login -l).
The vagrant images currently in use in the nightly tests
are configured with selinux disabled. Add skipif marker when
selinux is disabled.
Fixes: https://pagure.io/freeipa/issue/7974
Reviewed-By: François Cami <fcami@redhat.com>