add missing SPDX header to new tests

dune/istl/test/matrixfreesolverfactorytest.cc

+// SPDX-FileCopyrightText: Copyright © DUNE Project contributors, see file LICENSE.md in module root
+// SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
 // -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 // vi: set et ts=4 sw=2 sts=2:
 #include "config.h"

dune/istl/test/registermacrosbackwardcompatibilitytest.cc

+// SPDX-FileCopyrightText: Copyright © DUNE Project contributors, see file LICENSE.md in module root
+// SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
 #include <config.h>
 
 #include <dune/common/parametertree.hh>