"Outbound federation can request missing events" is flakey

https://buildkite.com/matrix-dot-org/synapse/builds/9760#b9dfca8e-8140-4db8-8b00-b2c2d2b01f11:

```
# Started: 2020-06-18 15:50:09.349
# Ended: 2020-06-18 15:50:10.258
# Got $159249541082Zmmlu:localhost:8831, expected $18:localhost:33251 for earliest_events[0] at tests/50federation/33room-get-missing-events.pl line 65.
# 0.135820: Registered new user @anon-20200618_154046-712:localhost:8831
```

Start with the Buildkite failure and tests/50federation/33room-get-missing-events.pl at line 65, then inspect how the expected earliest_events value is produced. Reproduce the failure and identify why the test sometimes receives a different event; done means the test reliably validates outbound federation missing-event requests without flaking.

Written by the indexing model from the issue text.