Make restricted room join tests clearer

[morguldir]

Firstly this PR makes the tests work on more implementations, for example ruma and GMS won't really understand join_authorised_via_users_server not looking like a user id, similar to what https://github.com/matrix-org/complement/pull/224 tried to do

Making it a user id that isn't correct still makes sure that the server can handle clients updating the member event directly with /myroomnick or similar

Some errors were also silently being ignored, which made it harder to tell which part of the test was actually failing

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