[Cashtab] Pass localforage as a param in cashtab test initializer function

Summary:
localforage is failing to clear properly across tests as it is initializing different instances in test file and helper function.

pass as a param so we have one instance that is easily cleared between tests

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15514