test_ldap.py: Add test for filter_{users,groups}
Related:
https://pagure.io/SSSD/sssd/issue/3362
Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
Reviewed-by: Pavel Březina <pbrezina@redhat.com>
(cherry picked from commit e1052a50b9cf42963e0a805a43e2957426096938)