[Cashtab] Create a standardized wrapper for test rendering

Summary:
Depends on D15429

Create a standardized wrapper to make it easier to add and edit tests (and make sure they are all testing the same thing).

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15431