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/server/sockfilt.c

@@ -21,7 +21,7 @@
  * SPDX-License-Identifier: curl
  *
  ***************************************************************************/
-#include "curl_setup.h"
+#include "first.h"
 
 /* Purpose
  *
@@ -85,23 +85,6 @@
  * it!
  */
 
-#ifndef UNDER_CE
-#include <signal.h>
-#endif
-#ifdef HAVE_NETINET_IN_H
-#include <netinet/in.h>
-#endif
-#ifdef HAVE_NETINET_IN6_H
-#include <netinet/in6.h>
-#endif
-#ifdef HAVE_ARPA_INET_H
-#include <arpa/inet.h>
-#endif
-#ifdef HAVE_NETDB_H
-#include <netdb.h>
-#endif
-
-#include <curlx.h> /* from the private lib dir */
 
 /* buffer is this excessively large only to be able to support things like
   test 1003 which tests exceedingly large server response lines */