Temporarily skip XClipboardSyncTest

Summary:
The test fails for unknown reasons on build.kde.org, but passes when
run on a development setup. As the code is going to be changed with
D15063 anyway, it is better to just disable the test for the time being.

Test Plan: Test gets skipped

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16895