[Cashtab] Improve useWalletStorage tests

Summary:
T3444

Mock the price API fetch for all tests (otherwise rendering the hook will call the actual API). Standardize format for tests of cashtabState. Improve spacing.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15508