tidy-up: minor code fixes and improvements

- schannel: drop redundant parentheses.
- os400sys: drop redundant includes.
  Follow-up to ebc5212dac #22374
- pytest: replace `()` with `[]` to match rest of tests.
- libtests: constify some local pointers.
- libtests: drop redundant `(long)` casts.
- lib650: use `CURL_CSTRLEN()`.
  Follow-up to 59dc2bbe07 #22424

Closes #22444

projects/OS400/os400sys.c

@@ -28,8 +28,6 @@
 #include <curl/curl.h>
 #include "config-os400.h"  /* Not curl_setup.h: we only need some defines. */
 
-#include <sys/types.h>
-#include <sys/socket.h>
 #include <sys/un.h>
 
 #include <stdlib.h>