[Cashtab] Improve flakiness in orderbook tests
Summary:
The min amounts are calculated, so can have a race condition here.
Use await screen.findByText for values that are expected to appear after async.
Test Plan: npm test
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D17090