[ecash-herald] Fix legacy chronik client mock import

Summary: A script is importing a nonexistent chronik mock. Point it to the new monorepo chronik mock.

Test Plan: npm test, npm run generateMock, grep -r '../test/mocks/chronikMock' and no result

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14723