Fix memory leak in tst_qtreewidget

Invalid inserts of items are tested. However, on an invalid insert
of QTreeWidgetItem into QTreeWidget, the QTreeWidget does not take
ownership of the to-be-inserted items. These items were leaked in
the past.
This patch introduces a scope-guard, so the items are not leaked.

Change-Id: Ib53ac02605c3a3c0bd1ae69bbd0876f860d08093
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

tests/auto/widgets/itemviews/qtreewidget/tst_qtreewidget.cpp

@@ -5,6 +5,7 @@
 #include <QApplication>
 #include <QHeaderView>
 #include <QLineEdit>
+#include <QScopeGuard>
 #include <QScrollBar>
 #include <QSignalSpy>
 #include <QStyledItemDelegate>
@@ -322,6 +323,7 @@ void tst_QTreeWidget::addTopLevelItem()
     // invalid insert
     {
         tops.clear();
+        const auto sg = qScopeGuard([&] { qDeleteAll(std::exchange(tops, {})); });
         for (int i = 0; i < 10; ++i)
             tops << new TreeItem();
         int count = tree.topLevelItemCount();