tests: drop unused or redundant includes

Closes #17717

tests/client/first.h

@@ -38,9 +38,6 @@ extern const struct entry_s s_entries[];
 
 #include "curlx.h"
 
-#include <stdlib.h>  /* for calloc(), free(), strtol() */
-#include <string.h>  /* for strchr(), strcmp() */
-
 #define ERR()                                                                 \
   do {                                                                        \
     curl_mfprintf(stderr, "something unexpected went wrong - bailing out!\n");\

tests/client/h2_pausing.c

@@ -25,8 +25,6 @@
  */
 #include "first.h"
 
-#include <assert.h>
-
 static void usage_h2_pausing(const char *msg)
 {
   if(msg)