[herald] Unit tests for parseWebsocketMessage

Summary:
T2972

Depends on D13616

Add unit tests for parseWebsocketMessage

These unit tests required a mock Telegram bot and improvements to chronikMock.js

For now, I think it's okay to maintain chronikMock.js as distinct files in both alias-server and ecash-herald. The incremental work in both apps is a good way to learn what features this mock needs. It will eventually be extended to Cashtab and live as a centralized dependency.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13617