test_schema_plugin: Drop dependency on Tracker
    
    Tracker is the best for testing plugins dealing with LDAP.
    The tests in test_schema_plugin are not used LDAP at all.
    
    Fixes: https://pagure.io/freeipa/issue/8954
    Signed-off-by: Stanislav Levin <slev@altlinux.org>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>