[autotests] Hopefully stabilise TestShellClient
Summary:
Apparently this test is flaky, based on the output I recieved we had
processed the un-fullscreen but not the un-maximise.
It seems possible the wayland thread could process when there's only one
request in the queue.
Test Plan:
Couldn't reproduce the original issue, it's a blind patch but the
current tests still pass.
Reviewers: KWin
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D16955