[Cashtab] Wrap useWallet hook test in act

Summary:
Getting away without this now but these tests will fail after upgrading React and testing libraries to latest versions

Use correct syntax and wrappers now in prep for upgrade

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15340