[Cashtab] Upgrade signverifymsg tests

Summary:
T3444

Add placeholder text to fields that could use it, and use that to get text inputs instead of data-testid. Implement standardized cashtab test wrapper.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15567