[ecash-lib] Fix test suite: Don't use it.only

Summary: Using it.only prevents all other tests from being run. Instead, it should be used so the full test suite is run.

Test Plan: npm run test

Reviewers: bytesofman, #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16529