Previously this manually initialized two sessions through an
AxolotlKit-provided helper. Rather than expose a similar helper for
SignalClient, TestProtocolRunner now sends an empty message in both
directions to initialize the sessions.
Tests from `receivedEnvelopeData`, which is called by e.g. the websocket or the
messageFetcherJob through the message processing pipeline until the UIDatabase
is notified with the new message and thread.
Includes some test helpers to initialize sessions and encrypt/decrypt messages.
