[cashtab-components] More unit tests for helper functions

Summary: Add unit tests for helper functions used by app in current state. Up next: strip all functions only relevant to legacy badger components, then complete unit test coverage, then automate publishing.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9235