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-execute/exec-execsearchpath-environment.service

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
 [Service]
 ExecStart=/bin/sh -x -c 'test "$$VAR1" = "word1 word2" && test "$$VAR2" = word3 && test "$$VAR3" = "\\$$word 5 6" && test "$$PATH" = "/tmp:/bin"'
 Type=oneshot