StyleSheet baseline test for TreeView: include one with empty area

So far, the entire tree view was filled with items, so we didn't see how
the empty area below the last item was rendered. Take a screen shot of
the tree after giving it twice the space it originally has to get some
empty area included as well, without breaking existing baseline images.

Pick-to: 6.8
Task-number: QTBUG-123632
Change-Id: I50714c0b3b04f4b1844e52063d0c0c77ff2d7154
Reviewed-by: Christian Ehrlicher <ch.ehrlicher@gmx.de>

tests/baseline/stylesheet/tst_baseline_stylesheet.cpp

@@ -206,6 +206,9 @@ void tst_Stylesheet::tst_QTreeView()
 
     tw->topLevelItem(Children)->child(0)->setSelected(true);
     QBASELINE_CHECK_DEFERRED(takeSnapshot(), "itemSelected");
+
+    testWindow()->resize(testWindow()->size() * 2);
+    QBASELINE_CHECK_DEFERRED(takeSnapshot(), "withEmptyArea");
 }
 
 void tst_Stylesheet::tst_QHeaderView_data()