[ecash-lib] Allow use of .only in mocha test files

Summary: Real pain to keep deleting and re-adding this when working on the repo. Now that we are using eslint flat config, arc lint will catch any lingering .only use

Test Plan: Add .only to a test file in ecash-lib, run arc lint, get a lint error, remove it, all good, npm test with a test using .only and only that test runs

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D17782