[cashtab] Fix the unit tests verbosity

Summary: Don't print to the console unless there is actually a test failure. The current unit test output is unusable because it's bloated with expected failures that we don't need/want to see.

Test Plan:
Be prepared, you could get shocked:

npm run test

Add a deliberate test failure and check it still pops up.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17456