Change exception SPDX to Autoconf-exception-generic

License text is in fact Autoconf generic exception, with already defined
SPDX identificator. Use that instead.

https://spdx.org/licenses/Autoconf-exception-generic.html
(cherry picked from commit 2b348a5daa)

bin/tests/system/custom-test-driver

@@ -6,7 +6,7 @@ scriptversion=2021-09-20.08 # UTC
 
 # Copyright (C) 2011-2020 Free Software Foundation, Inc.
 #
-# SPDX-License-Identifier: GPL-2.0-or-later WITH LicenseRef-Automake-exception-2.0
+# SPDX-License-Identifier: GPL-2.0-or-later WITH Autoconf-exception-generic
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by