tst_qsglistview::currentIndex unreliable

The tst_qsglistview::currentIndex test is unreliable.

It appears that the problems occur when the final step in the smoothed animation running on the ListView is a change of < 1.0. In this case QSGListView::trackedPositionChanged() does not move the list.

The whole QSGListView class contains a bunch of +1 and -1 adjustments that seem fishy to me - maybe they worked when we used more integer math, but they seem to be failing now.