[Cashtab] Remove instance of toSatoshi from tx builder

Summary: Continuing the refactor of the sendBch function, remove the 2nd (of 3) instances of the toSatoshi function. Add unit tests to confirm that the new method is returning identical output to the old method.

Test Plan: npm test

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9166