This is only 'mostly' correct currently, because what should be no-op dupe
joins will actually trigger the entire room state to be re-sent.
Bizarrely, it's significantly easier to just do that than work out if we should,
and there are no client-visible effects to doing so, so we just do it for now.
- Test data for the sync server is now in its own file.
- Rejig the sync server tests to support multiple /sync requests and corresponding
assertions.
- Fixed a minor bug which resulted in state events to appear twice in /sync
responses when syncing without a `since` parameter.
