[Cashtab] Bump timeout in test with expectation of load time

Summary:
Seeing this failure in prod on this specific line with this comment, just bump the timeout.

We are loading multiple tokens in the background. The test is confirming that this does happen, tho it is not instant and is not expected to be instant.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18035