various: mark files with the usual license tags

For .clang-format and pytest.ini, MIT-0 is used because people might want
to copy those.

test/test-sysusers/test-00-basic.input

@@ -1,9 +1,4 @@
-#  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
 
 # The superuser
 g root    0       -            -