[Cashtab] Remove the watchAll flag from the test script

Summary:
Cashtab has so many tests now and they take time to run. It is not common where we want to keep iteratively running the tests while files change. Or, at least, this is not the most common case.

By default, we should not watchAll

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18630