Add missing wait for window shown.

Change-Id: I746a12dfa1b87a0d1393341ca3d641f73bcaee0a
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>

tests/auto/widgets/itemviews/qtreeview/tst_qtreeview.cpp

@@ -2017,6 +2017,8 @@ void tst_QTreeView::clicked()
     view.setModel(&model);
     view.show();
 
+    QVERIFY(QTest::qWaitForWindowExposed(&view));
+
     QModelIndex firstIndex = model.index(0, 0, QModelIndex());
     QVERIFY(firstIndex.isValid());
     int itemHeight = view.visualRect(firstIndex).height();