[herald] Add unit tests to CI

Summary:
T2972

Following the model of alias-server, add unit ecash-herald unit tests to CI.

Test Plan: Call the bot to run the CI

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13569