[Cashtab] Apply standardized test wrappers to App.test.js

Summary: As a precursor to combining the app integration tests into one file, update the original tests to use new standardized wrappers

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15474