qt: fix broken unit tests on MacOS

Summary:
I broke unit tests for Mac OS in D18200 with a typo. Fix it here.

Also include missing header needed for the qWarning() << syntax. It does not seem to be required, but it doesn't hurt to include it (it is included in every other cpp file that uses qWarning() << ...

Test Plan:
ninja all check-all

build-native-osx job on CI

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18204