Unset all env variables related to KDE session in integration tests

Summary:
When running ctest in my session a few OpenGL/waylandonly tests crashed
on tear down. This does neither happen on build.kde.org nor when running
ctest in a tty. Comparing the env variables of tty and session pointed
to the session variables. Unsetting those makes the test not crash. This
makes sense as e.g. plasma-integration no longer gets loaded.

As our test suite is intended to test KWin and not plasma-integration or
gnome integration we should have a clean and reproducable environment,
so the variables are unset.

Test Plan: 100% tests passed, 0 tests failed out of 130

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16939