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 2c27a67daa #17590

Closes #17628

scripts/mk-unity.pl

@@ -91,10 +91,10 @@ foreach my $src (@src) {
     if($src =~ /([a-z0-9]+)\.c$/ && !exists $exclude{$src}) {
         my $name = $1;
         my $fn = $src;
-        if($srcdir ne "" && (exists $include{$src} || $embed) && -e "$srcdir/$fn") {
-            $fn = $srcdir . "/" . $fn;
-        }
         if($embed) {
+            if($srcdir ne "" && -e "$srcdir/$fn") {
+                $fn = $srcdir . "/" . $fn;
+            }
             my $content = do { local $/; open my $fh, '<', $fn or die $!; <$fh> };
             print $content;
         }