Fix flaky QQmlProfilerService::scenegraphData test

QSGRenderThread::syncAndRender first triggers the frameSwapped() signal,
and only then adds the SceneGraphRenderLoopFrame message. We're waiting
for the frameSwapped signal to stop profiling, and therefore can't be sure
whether we'll get the final SceneGraphRenderLoopFrame message or not.

Task-number: QTBUG-33024
Change-Id: I3216667e8e5cdc79f210290ce22cc6b08c579b6a
Reviewed-by: Lars Knoll <lars.knoll@digia.com>