[Cashtab] Constant locale for jest snapshots

Summary:
Cashtab is designed to render some values based on the user's browser locale. Snapshot tests should use a constant locale to get a constant result.

This fix existed before the webpack upgrade. Was not carried through.

Test Plan: From France, npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14111