[Cashtab] Remove final instance of toSatoshi()

Summary: Replace the last instance of toSatoshi() with the new toSmallestDenomination() to support possible decimal move. Includes unit test demonstrating replaced value is equivalent to former value. This is the last step required before refactoring sendBch() to only use BigNumber

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9173