[Cashtab] [Cashtab] [final bch-api deprecation p7] Use real chronik URLs

Summary:
T2730, T2735

Depends on D12793

Replace fake URLs used in testing with real chronik URLs. Cashtab defaults to the ABC-managed chronik.fabien.cash, and falls back to chronik.be.cash

Test Plan:
Introduce an error into currency.chronikUrls[0], i.e. change https://chronik.fabien.cash to https://chronik.fabien.cas
npm start
Watch app self correct at the backup API

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12794