[tg bot] Improve how mocks are generated and stored

Summary:
T2972

Depends on D13623

It's been a persistent headache that generateMocks uses slightly different syntax / functions than what is used in the app. Some differences are necessary to preserve blockNames, which are useful in developer review and keeping track of why some blocks are mocked. But, in general, we want to be sure that the mocks are generated in exactly the same way that the app is running.

This diff refactors generateMocks to use the new handleBlockConnected function, modified with an optional flag to return mocks.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13624