qquicklistview autotest: flickBeyondBounds fails on CI system only

After change Ia613ddb25f46d452603f7cb567b74e80c95a346c (see also QTBUG-22407), the behavior is intentionally changed so that flicking beyond bounds has a more "springy" feel. So it's expected that you might have to flick "harder" to get two rows of a ListView completely out of bounds such that the delegates get destroyed. The test was only dragging 1000 pixels. Increasing it to 2000 makes it pass reliably, yet it still fails on the CI system only. There is no good explanation for that yet.