Sytest was using a wrong `history_visibility` for `invited`
(https://github.com/matrix-org/sytest/pull/1303), so `invited` was
passing for the wrong reason (-> defaulted to `shared`, as `invite`
wasn't understood).
This change now handles missing events like Synapse, if a server isn't
allowed to see the event, it gets a redacted version of it, making the
`get_missing_events` tests pass.
3c1474f68f