[Cashtab] Add unit tests for send BCHA amount validation

Summary: Add a validation.js file in utils to split form validation out from functional component and allow efficient unit tests. Unit tests for covered error cases.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9021