[mock-chronik-client] Support ws.unsubscribe

Summary:
Add overlooked ws.unsubscribe feature to mocked-chronik-client

Clean up ws unit tests which, through bugs, made it look like this feature existed.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15637