Use it from libtests' `first.c` and thus also from units, and tunits.
Also:
- cmake: drop stray `curltool` lib dependency for units.
- units: stop depending on `src` headers.
- tests/server: drop depending on `src` headers.
(the remaining one listed in the comments, `tool_xattr.h`, was not
actually used from servers.)
- tests/server: drop duplicate curlx headers.
(Except `warnless.h`, which is tricky on Windows.)
Closes#17672
