[autotests] Destroy main window in ToplevelOpenCloseAnimationTest::testDontAnimatePopups

Summary: Be nice and destroy all created windows in testDontAnimatePopups.

Test Plan: The test still passes.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18037