Suspected flake: `sync_should_succeed_even_if_the_sync_token_points_to_a_redaction_of_an_unknown_event`

[DMRobertson]

https://github.com/matrix-org/synapse/actions/runs/3051769294/jobs/4920425018

If this is a Synapse bug rather than a test issue, we should move it to the Synapse repo.