build: split `.c` and `.h` file lists in tests

build: split `.c` and `.h` file lists in tests

It allows to pass `.h` files only where they are really needed, which is
`EXTRA_DIST` (or `*SOURCES`) for autotools.

Also:
- rename variables to be shorter and consistent.
- drop references to non-local headers. We don't need to pass them as
  dist sources.
- drop empty variables and references, after the above.
- server: add placeholder `UTIL_H`.

Closes #17745