[herald] Improve sendTestTgMsgs script

Summary:
T2972

Depends on D13639

Rename script to match filename. Add comments for script usage. Patch script to accept new blocks format.

Test Plan: npm run sendTestTgMsgs

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13640