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>

LICENSES/LicenseRef-old-glib-tests.txt

@@ -0,0 +1,16 @@
+This work is provided "as is"; redistribution and modification
+in whole or in part, in any medium, physical or electronic is
+permitted without restriction.
+
+This work is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+
+In no event shall the authors or contributors be liable for any
+direct, indirect, incidental, special, exemplary, or consequential
+damages (including, but not limited to, procurement of substitute
+goods or services; loss of use, data, or profits; or business
+interruption) however caused and on any theory of liability, whether
+in contract, strict liability, or tort (including negligence or
+otherwise) arising in any way out of the use of this software, even
+if advised of the possibility of such damage.