`TestFederatedEventRelationships` seems flakey

From: https://github.com/matrix-org/complement/issues/568

`TestFederatedEventRelationships` seems to turn up repeatedly in CI flakes;
- https://github.com/matrix-org/complement/actions/runs/3496172602/jobs/5853777083
- https://github.com/matrix-org/complement/actions/runs/3497977607/jobs/5857756804