[autotests] Handle async maximisation in quick tiling test

Summary:
KWin now correctly updates the client maximised state when we have a new
buffer in the new state. The quick tile unit test was unfortunately not
updated.

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: graesslin, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16739