[Cashtab] split out backend functions from tests

Summary: The getSlpBalancesAndUtxos function from useBCH.js included an API request. This diff moves the API request to a separate function, getHydratedUtxoDetails. It also removes the test for getUtxos as this is a pure API call. Tests were failing because the utxos at the mock addresses had been spent. Mocks updated to a more complicated utxo set.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8966