[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