test/35fa71a030ca: Fix -Wunused-but-set-variable warning from clang-15

clang-15 correctly warns:
```
  35fa71a030ca.c:241:7: error: variable 'iter' set but not used \
  [-Werror,-Wunused-but-set-variable]
    int iter;
        ^
  1 error generated.
  make[1]: *** [Makefile:210: 35fa71a030ca.t] Error 1
```
Remove the iter variable here, it's not used.

Fixes: 78f59df333 ("Add regression test cases for three recent issues")
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>

test/35fa71a030ca.c

@@ -238,8 +238,7 @@ static void execute_one(void);
 
 static void loop(void)
 {
-  int iter;
-  for (iter = 0;; iter++) {
+  for (;;) {
     int pid = fork();
     if (pid < 0)
       exit(1);