Flickable could start dragging sooner on a trackpad with pixelDeltas

tst_qquickflickable::trackpad() wasn't testing the right code path. After fixing that, it's surprising how many events it takes to finally get the Flickable moving (4 events? really?) And also surprising how that's not very noticeable in real life, because events come in pretty fast; but we don't have it as responsive as it could be, in comparison with native macOS components.