build: sync tests unity builds between cmake and autotools

build: sync tests unity builds between cmake and autotools

Instead of relying on CMake's built-in unity feature, use `mk-unity.pl`,
as already done with autotools. It simplified the build, shortens logs
and makes debugging easier because of the fewer build variations.
It also allows testing / fixing with cmake and those automatically apply
to autotools builds too. cmake builds can be much-much faster, esp.
when working the builds themselves.

It also enables "unity" in old cmake versions. Basically every test
target is a single generated .c source.

Also:
- drop a `lib` unity workaround for libtests with autotools after fixing
  the issue in libtests itself. It drops a few exceptions and makes
  libcurl build faster (in autotools unity).
- fix another `lib` autotools unity issue and drop the workaround for it
  from `mk-unity.pl`. `srcdir` was missing from the header path.
- simplify `mk-unity.pl` command-lines, drop exclusions.

Follow-up to 2c27a67daa1b76859c18d63e4e1f528db05b5e13 #17590

Closes #17628