tst_QPointer: make DerivedParent delete all children

... not just 'derivedChild'.

This doesn't change anything for the test right now, but will enable
an extended test to be added on top.

Pick-to: 6.9 6.8 6.5 5.15
Change-Id: Id52f785168c97c43433fb15a6a71a9d1fb140036
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/corelib/kernel/qpointer/tst_qpointer.cpp

@@ -3,6 +3,8 @@
 
 #include <QTest>
 #include <QtTest/private/qcomparisontesthelper_p.h>
+
+#include <QtCore/private/qobject_p.h>
 #include <QRunnable>
 #include <QThreadPool>
 
@@ -383,7 +385,7 @@ DerivedParent::DerivedParent()
 
 DerivedParent::~DerivedParent()
 {
-    delete derivedChild;
+    QObjectPrivate::get(this)->deleteChildren(); // like ~QWidget() does
 }
 
 DerivedChild::~DerivedChild()