Autotests/QItemView: re-enable tst_QItemView::indexAt()

tst_QItemView::indexAt() was disabled since Qt 5.0 (and maybe earlier)
maybe due to it's long runtime (15s on my machine).
Speed it up by checking only some special positions inside the visual
rect (borders, center) as it will likely not fail on other positions but
succeed for the ones which get tested.

Task-number: QTBUG-22470
Change-Id: I5c7135757049176f9daca4afc1b7f40c75b9ecd9
Reviewed-by: David Faure <david.faure@kdab.com>