Suspected flakiness in New federated private chats get full presence information (SYN-115)

[DMRobertson]

https://github.com/matrix-org/synapse/runs/4067253219?check_suite_focus=true had this test fail in 1/6 sytest runs.

I wondered if the changes in #1160 had unintentionally introduced some kind of flakey problem. But the failure in that link does not arise from running sytest on synapse with workers (the usual suspect for for flakiness). That was a surprise.

Raising this so we can track any future sightings.

[DMRobertson]

https://github.com/matrix-org/sytest/runs/4091934296?check_suite_focus=true

[anoadragon453]

https://github.com/matrix-org/synapse/runs/4224174216?check_suite_focus=true

[DMRobertson]

https://github.com/matrix-org/synapse/runs/4239643028?check_suite_focus=true

[richvdh]

per matrix-org/synapse#1663, this seems to have been known for a long time, but I'm not sure why it suddenly flared up again?

[anoadragon453]

https://github.com/matrix-org/synapse/runs/4870079986?check_suite_focus=true

[DMRobertson]

https://github.com/matrix-org/synapse/runs/5986096002?check_suite_focus=true

[DMRobertson]

https://github.com/matrix-org/synapse/runs/6078594145?check_suite_focus=true

[babolivier]

https://github.com/matrix-org/synapse/runs/7444165977?check_suite_focus=true

[babolivier]

https://github.com/matrix-org/synapse/runs/8112053299?check_suite_focus=true