[Cashtab] Update tests and patch bug in SendByUrlParams.test.js

Summary:
T3444

Implement standardized cashtab test wrapper. Testing revealed bug where input fields were being set before balance had updated, leading to incorrect validation errors.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15568