tests: add spdx license header to test unit/link/network/conf files

Those are all consumed by our parser, so they all support comments.
I was considering whether they should have a license header at all,
but in the end I decided to add it because those files are often created
by copying parts of real unit files. And if the real ones have a license,
then those might as well. It's easier to add it than to make an exception.

test/test-path/basic.target

@@ -1,11 +1,4 @@
-#  SPDX-License-Identifier: LGPL-2.1-or-later
-#
-#  This file is part of systemd.
-#
-#  systemd is free software; you can redistribute it and/or modify it
-#  under the terms of the GNU Lesser General Public License as published by
-#  the Free Software Foundation; either version 2.1 of the License, or
-#  (at your option) any later version.
+# SPDX-License-Identifier: LGPL-2.1-or-later
 
 [Unit]
 Description=Basic System