[Cashtab] Move app tests to appropriate dir level

Summary:
App.js is at the top level of src/components. It's test should also be at the top level, at src/somponents/__tests__

It is instead at src/components/Common/__tests__. This is the result of an error when creating directories.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15333