[Cashtab] Bump jest timeout to 30s

Summary: Some tests, esp the combined test of Configure.test.js, are expected to take this kind of time. Has not been an issue (yet) in CI, but do occasionally hit it locally.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15478