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

[vszakats]

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.

---

• still thinking about variable names, because _H conflates
  with feature check macros and header guards, and messes up
  greps. → It's only FIRST_H and UTILS_H, so this should be
  fine.
• there is room to improve consistency in internal variables
  with src and libs. Future PR.