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.
Previously, event if link's setup state is in failed, tests may pass,
as systemd-networkd-wait-online success if the state is in failed state.
This makes tests be checked more strictly.