Center/position windows in tst_qmenu.

Avoid menus showing up at 0,0 which is a taskbar area when running
Unity.

Task-number: QTBUG-33972
Change-Id: I156eec78248cec1708adf6bcf2e9ddcc98b7d8c4
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>