tst_qidentityproxymodel: map the index to the source model

In data(), index belongs to the proxy, not to the source. It needs
to be mapped back to it first.

Change-Id: Ie5dcbf13166dadf62f3d85b594d3227383132521
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

tests/auto/corelib/itemmodels/qidentityproxymodel/tst_qidentityproxymodel.cpp

@@ -360,7 +360,7 @@ class AppendStringProxy : public QIdentityProxyModel
 public:
     QVariant data(const QModelIndex &index, int role) const
     {
-        const QVariant result = sourceModel()->data(index, role);
+        const QVariant result = QIdentityProxyModel::data(index, role);
         if (role != Qt::DisplayRole)
             return result;
         return result.toString() + "_appended";