[Cashtab] Add eslint plugin for react testing library

Summary:
Ref https://kentcdodds.com/blog/common-mistakes-with-react-testing-library

Add recommended linter for best practices with react testing library

Fix some false-positive test passes uncovered by linter.

Test Plan: eslint . and no output, npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15393