[Cashtab] Move file not in right dir

Summary:
Wallet test file in wrong place. Highlights that the convention of even using __test__ dir is obsolete and needs to be dropped. But that should be done in a separate diff that fixes it everywhere.

For now, stick to the existing standard.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15255