[Cashtab] Run tests in UTC timezone

Summary:
T2346

This feature is already in Cashtab, needs to be copied over here to the upgraded test script in new CRA base

Test Plan: npm test and all tests pass

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11263