Adjust for feedback from PR#18999

[jason-famedly]

Requires #6

[jason-famedly]

Fascinating that the trial psycopg test had a fluke failure:

[FAIL]
Traceback (most recent call last):
  File "/home/runner/work/synapse/synapse/tests/rest/admin/test_user.py", line 5785, in test_redact_messages_all_rooms
    self.fail("Timed out waiting for redactions.")
twisted.trial.unittest.FailTest: Timed out waiting for redactions.

tests.rest.admin.test_user.UserRedactionBackgroundTaskTestCase.test_redact_messages_all_rooms

It appears this test is utilizing time.time() to check the timeout. Unfortunately, this is considered a flake as using fake time and real time in a test is automatically suspect when any kind of contention is happening(and is beyond the scope of what we are trying to accomplish)