[Cashtab] Convert NFT tests to ts

Summary: Continue incremental ts conversion of Cashtab tests. We redefine some mocks to make mocking easier using the new prepareContext function. We remove mocks that are no longer used.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18629