[herald] Unit tests for initializing websocket

Summary:
T2972

Depends on D13615

Refactor websocket functions to accept chronik and telegramBot as params, as these must be mocked in unit tests.

Add unit tests for initializing the websocket.

Note: additional unit tests to follow later in this stack

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13616