Plug memleak in tst_QStackedWidget

To keep the change minimal, keep 'sw' as a pointer variable,
but back it by a stack-allocated QStackedWidget instead of a
heap-allocated one that's never deleted.

Change-Id: I9e2a8c07979b861eb7e7040c144d8e75c90d0bc9
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

tests/auto/widgets/widgets/qstackedwidget/tst_qstackedwidget.cpp

@@ -168,7 +168,8 @@ private:
 
 void tst_QStackedWidget::dynamicPages()
 {
-    QStackedWidget *sw = new QStackedWidget;
+    QStackedWidget stackedWidget;
+    QStackedWidget *sw = &stackedWidget;
 
     TestPage *w1 = new TestPage(true);
     w1->setN(3);