tst_QComboBox: fix popupPositionAfterStyleChange() FAIL on Kubuntu 20.04

On my machine, the window manager opened the QComboBox always at y = 0
(top edge of the screen), where the popup, being confined to the
screen geometry, had no chance of "open[ing] up centered on top of the
current index".

Fix by centering the widget on screen.

Amends 8393922e7071221a9c6c0811eb714f20bf4ed02b.

Pick-to: 6.9 6.8 6.5
Change-Id: I7b4d066aa1e555f3d5cc1d5e0c88b1d835bf2b26
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

tests/auto/widgets/widgets/qcombobox/tst_qcombobox.cpp

@@ -3490,6 +3490,7 @@ void tst_QComboBox::popupPositionAfterStyleChange()
         QSKIP("Flaky on QEMU, QTBUG-114760");
 
     box.addItems({"first", "middle", "last"});
+    centerOnScreen(&box);
     box.show();
     QVERIFY(QTest::qWaitForWindowExposed(&box));
     box.showPopup();