[Cashtab] Upgrade to React 18

Summary:
Upgrade to React 18. This allows us to better maintain the dependency tree and take advantage of the latest testing libraries.

Bump jest timeout as, althought tests passed in first CI here, were flaky locally.

Add waitFor on flaky local test (available method in upgraded library).

The react hooks testing library is now part of @testing-library/react. Must use this version with React 18. So, the legacy dependency is removed, and @testing-library/react is upgraded.

Test Plan:
npm test
npm run build

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15336