Add a comment to the class QPropagationTestWidget of tst_QWidget

It is not obvious, why this class has been added.

Add a comment to explain.

Change-Id: Ife19e7f508e23dc22968a462d2d00e6698b807b3
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp

@@ -893,6 +893,8 @@ void tst_QWidget::fontPropagation()
     QVERIFY(four->testAttribute(Qt::WA_SetFont));
 }
 
+// QPropagationTestWidget is not found in QApplicationPrivate::widgetPalettes
+// and therefore falls back to the system palette
 class QPropagationTestWidget : public QWidget
 {
     Q_OBJECT