[Cashtab] Bring QRCode tests in line with rest of react testing lib tests

Summary:
Existing tests here are not particularly helpful...expecting onClick to have been called on a click.

Update so we test the actual behavior we want to see -- user clicks, a div is displayed that has the text content of the address.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15480