[ecash-herald] patch bug in satsToFormattedXec

Summary:
T2972

Depends on D14081

Add >= to make sure there isn't a blind spot in coverage of formatted values. Add unit test to confirm.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D14082