tests: mark xfail for test_selinux_user_optimized on fed<=28
    
    The test TestUserPermissions::test_selinux_user_optimized is
    testing the fix for SSSD issue 3819, but the fix is not
    available in fedora 28. Hence mark the test as xfail when
    executed on fedora <=28 (our nightly tests also run on fed 28).
    
    For full ref: fixed in sssd 1.16.4, Fedora 28 provides
    1.16.3-2.fc28 only, while Fedora 29 provides 2.0.0-3.fc29.
    
    related ticket : https://pagure.io/SSSD/sssd/issue/3819
    
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>