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

tests/libtest/lib1560.c

@@ -30,7 +30,7 @@
  * that this test will assume to be present!
  */
 
-#include "test.h"
+#include "first.h"
 #if defined(USE_LIBIDN2) || defined(USE_WIN32_IDN) || defined(USE_APPLE_IDN)
 #define USE_IDN
 #endif