Element-R: wait for OlmMachine on startup (#3487)
* Element-R: wait for OlmMachine on startup
Previously, if you called `CryptoApi.getUserDeviceInfo()` before the first
`/sync` request happened, it would return an empty list, which made a bunch of
the tests racy. Add a hack to get the OlmMachine to think about its device
lists during startup.
* add a test
