Unflake "Server correctly resyncs when server leaves and rejoins a room"

[anoadragon453]

This test flaked on me here with the following sytest output:

# Started: 2020-06-02 16:11:09.325
# Ended: 2020-06-02 16:11:10.785
# HTTP Request failed ( 404 Not Found https://localhost:8831/_matrix/client/r0/join/!PkvlNjehVHBRXtsZBJ:localhost:8831?server_name=localhost%3A44489&access_token=MDAxY2xvY2F0aW9uIGxvY2FsaG9zdDo4ODMxCjAwMTNpZGVudGlmaWVyIGtleQowMDEwY2lkIGdlbiA9IDEKMDAzYmNpZCB1c2VyX2lkID0gQGFub24tMjAyMDA2MDJfMTU1NjM1LTc0Mzpsb2NhbGhvc3Q6ODgzMQowMDE2Y2lkIHR5cGUgPSBhY2Nlc3MKMDAyMWNpZCBub25jZSA9ID1EaCs2bUh0SEhRWThTQnMKMDAyZnNpZ25hdHVyZSCKQJ7oXgJRqAE6WliaMy8nk7YBCL8U_xChZStw-JPdxAo ) from POST https://localhost:8831/_matrix/client/r0/join/!PkvlNjehVHBRXtsZBJ:localhost:8831?...
# {"errcode":"M_NOT_FOUND","error":"Unknown room !PkvlNjehVHBRXtsZBJ:localhost:8831"}
# 0.106658: Registered new user @anon-20200602_155635-743:localhost:8831
# 0.922439: Joined room 1: !PkvlNjehVHBRXtsZBJ:localhost:8831
# 1.007631: first query response
# {
#   device_keys       => {
#                          "\@__ANON__-64:localhost:44489" => { random_device_id1 => { device_keys => {} } },
#                        },
#   failures          => {},
#   master_keys       => {},
#   self_signing_keys => {},
#   user_signing_keys => {},
# }

Looks like it was due to the room join not completing before we try to manipulate the room. Add retry_until_success to ensure we join the room before continuing further.