[tg bot] Improve mock generation

Summary:
T2972

Depends on D13415

Improve mock generation with addition of new script npm run newmocks that overwrites blocks.js

Test Plan:
npm run newmocks
npm test and pass
npm run msgs

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D13416