tst_QItemDelegate: try to stabilize combobox test

tst_QItemDelegate::comboBox() is flaky because sometimes the used
QTableWidget does not yet have the focus which prevents a correct
editing. Fix it by explictily setting the focus on the widget after it
is shown.
A similar fix was added for dateTimeEditor() in
9822d57d85.
With this patch the test no longer fails for my on opensuse, therefore
remove the blacklisting.

Task-number: QTBUG-67282
Change-Id: I907db662ca347f8e8d31e5be215a100377b159ca
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Kari Oikarinen <kari.oikarinen@qt.io>

tests/auto/widgets/itemviews/qitemdelegate/BLACKLIST

@@ -2,7 +2,3 @@
 opensuse-42.3 ci
 [testLineEditValidation]
 opensuse-42.3 ci
-[comboBox]
-# QTBUG-67282
-opensuse
-opensuse-leap

tests/auto/widgets/itemviews/qitemdelegate/tst_qitemdelegate.cpp

@@ -1400,6 +1400,7 @@ void tst_QItemDelegate::comboBox()
     widget.setItem(0, 0, item1);
     widget.show();
     QVERIFY(QTest::qWaitForWindowExposed(&widget));
+    QApplication::setActiveWindow(&widget);
 
     widget.editItem(item1);