tests: make individual test sources compile cleanly

Tidy up headers and includes to ensure all individual test source
compile cleanly (but not link). To allow running clang-tidy (and
possibly other static analyzers) on them. It also improves readability
and allows to verify them locally, without the bundle logic.

clang-tidy ignores #included C files, so it's blind to bundle C files
the include these tests. The current workaround of embedding has
a couple of downsides:. meaningless filenames and line numbers,
missing issues, messing up self header paths. Thus, running it on
individual sources would be beneficial.

Also:
- de-duplicate includes.
- untangle some includes.
- formatting/indentation fixes.
- merge `getpart.h` into `first.h`.

Ref: https://github.com/curl/curl/pull/17680#issuecomment-2991730158

Closes #17703

scripts/mk-unity.pl

@@ -97,6 +97,6 @@ foreach my $src (@src) {
 }
 
 if($any_test) {
-    print "\nstatic const struct entry_s s_entries[] = {\n$tlist};\n";
+    print "\nconst struct entry_s s_entries[] = {\n$tlist  {NULL, NULL}\n};\n";
     print "\n#include \"first.c\"\n";
 }