Add a SPDX LicenseRef for the license historically used for tests

Some of GLib's unit tests are under an apparently GLib-specific
permissive license, vaguely similar to the BSD/MIT family but with the
GPL's lack-of-warranty wording. This is not on SPDX's list of
well-known licenses, so we need to use a custom license name prefixed
with LicenseRef if we want to represent this in SPDX/REUSE syntax.

Most of the newer tests seem to be licensed under LGPL-2.1-or-later
instead.

Signed-off-by: Simon McVittie <smcv@collabora.com>

glib/tests/rec-mutex.c

@@ -2,6 +2,8 @@
  * Copyright (C) 2011 Red Hat, Inc
  * Author: Matthias Clasen
  *
+ * SPDX-License-Identifier: LicenseRef-old-glib-tests
+ *
  * This work is provided "as is"; redistribution and modification
  * in whole or in part, in any medium, physical or electronic is
  * permitted without restriction.