Remove potential endless loop from XClipboardSyncTest
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
Differential Revision: https://phabricator.kde.org/D16741