Improvements to megolm integration tests (#3060)
The megolm tests were making a few assumptions which they really shouldn't; in
particular:
* They were creating mock events with event_ids not starting `$`, and lacking
`sender`, `origin_server_ts` and `unsigned` properties
* They were not including the (now) required `keys.ed25519` property inside
the ciphertext of an olm message.
These work ok currently, but they aren't really correct, and they cause
problems when testing the new rust implementation.
