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.
We usually disable IPv6AcceptRA= if the test does not require any
dynamic address configuration, as it makes slightly slow down the test.
C.f. 491b79aeac.