"guest user can set displaynames" is flaky

[richvdh]

eg https://buildkite.com/matrix-dot-org/synapse/builds/7348#f0749e42-bda8-48f9-95fb-3f862aa7fca4

# Started: 2020-02-26 09:55:08.990
# Ended: 2020-02-26 09:55:10.021
# Got 7, expected "creeper" for Room displayname at tests/30rooms/13guestaccess.pl line 190.
# 0.167456: Registered new user @anon-20200226_095239-167:localhost:8831
# 0.919001: User @7:localhost:8831 joined room
# { room_id => "!QgjZoiPzZxNvhctfuI:localhost:8831" }

[clokep]

I saw a couple more instances of this, both on worker runs.

The assertion that fails is:

sytest/tests/30rooms/13guestaccess.pl

Line 190 in 128d7d3

assert_eq( $body->{displayname}, "creeper", "Room displayname" );

[clokep]

Been able to reproduce this locally (although it seems to happen only 1 out of every 4 times or so):

docker run --rm -it -e POSTGRES=true -e WORKERS=true -v /Users/clokep/matrix/synapse\:/src:ro -v /Users/clokep/matrix/sytest/logs\:/logs -v /Users/clokep/matrix/sytest\:/sytest:ro matrixdotorg/sytest-synapse:py35 tests/30rooms/13guestaccess.pl

[clokep]

I attempted to fix this (see 5b1960d), but that seems to have caused a bunch of other tests to have gone flaky.

[clokep]

I think the failed tests on ^ were the other failures recently fixed. I rebased and that passed without issue.