[Cashtab] Patch CI by including mock-chronik-client

Summary:
D17499 broke CI because it introduces a non-test typescript file that uses mock-chronik-client. imo it's important to have this file, which exports functions that will be used to support standardized test frameworks throughout the app. We should not be exporting functions from test files. So the test dir is appropriate for this.

Surprised builds didn't fail before without this dep. But makes sense as mock-chronik-client only used in test files as a dev dep (still just a dev dep), and ts ignores the test files.

Test Plan: docker build -f cashtab.Dockerfile -t cashtab_local .

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17505