Improve tests behaviour on set ups with high dpi

Summary:
We are testing on a 100x100 mock display. We'd ask QWidget for our
display's dpi, it would not use the mock display but the actual
system's. On my system it made the corners overlap with each other and
all sorts of weird things happened.

Test Plan: The tests actually pass here.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D28224