TESTS: Fix E711 pep8 issues on sssd_group.py
    
    E711: comparison to None should be 'if cond is not None:'
    
    The issue was found on: debian_testing, fedora22, fedora23, rhel6 and
    rhel7 machines.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3605
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>