Fix the Qt tests on MacOS

Summary: We need to create the directories so the tests can find them. Not sure why it doesn't fail on windows and linux but there is quite a lot of platform dependent code in the path handling.

Test Plan: On macos: ninja check-bitcoin-qt

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18718