Follow-up for af718e0535
The commit changed the documentation, but the actual impl
was apparently not updated. And the documented behavior
feels a bit off. I think generally "auto"/true should
override $NO_COLOR.
Plus, the test for auto-24bit is at odds with the logic
we merged. I guess it was overlooked after applying
https://github.com/systemd/systemd/pull/40303#discussion_r2720450393
To make sure everything still compiles, we add a preliminary include
of forward.h to tests.h to make sure it is included in every test source
file. We'll clean up the tests.h includes in a later commit.
We also add a <errno.h> include to errno-list.h to keep test-errno-list.c
compiling. It'll be removed again when we clean up includes in src/basic.
Split out of #37344.
Ideally, we'd read back what we wrote, but that would have been
much more complicated. But just writing stuff is useful to test under
valgrind or manually.