Remove potential endless loop from XClipboardSyncTest

Summary:
We need to wait till the helper process created the datadevice. For this
we used a while loop. On build.kde.org the test gets stuck in this loop
and times out after 10 minutes.

This change introduces a dedicated signal and we just wait for it. So if
something fails we wait only 5 sec instead of endless. This should help
investigate why the test doesn't work on build.kde.org.

Test Plan: Test works locally

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16741