[Cashtab] Pass chronik as a prop to useWallet hook

Summary:
Pass chronik as a prop to useWallet.js hook. This enables full-app integration testing by passing mock chronik client.

Add a few simple unit tests to confirm this.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15381