[tg bot] Unit test for main()

Summary:
T2972

Depends on D13617

Unit test app starting function, main. chronikMock.js is improved to return more realistic websocket info.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13618