[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