[AIRFLOW-5746] move FakeDateTime into the only place it is used (#6416)
[AIRFLOW-5746] move FakeDateTime into the only place it is used (#6416)

Co-authored-by: Jarek Potiuk <f41fb3c6021305a2b88483c10bc35385d65a7c7a@polidea.com>
(cherry picked from commit adfcf67d6532350071336390bcb4dad0b99ae450)

owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow