tests: Replace returns with exits in the main functions

This will allow us to redefine the exit function.

For example:
```
#define exit(code) { \
    fprintf(stderr, "%s:%d: exit(%s) in function ‘%s’\n",
        __FILE__, __LINE__, #code, __func__); \
    _exit(code); \
}
```

tests/string/mem.c

@@ -9,7 +9,7 @@ int main(void) {
     arr[50] = 1;
     if ((size_t)memchr((void *)arr, 1, 100) - (size_t)arr != 50) {
         puts("Incorrect memchr");
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     }
     puts("Correct memchr");
     char arr2[51];
@@ -17,25 +17,25 @@ int main(void) {
     memccpy((void *)arr2, (void *)arr, 1, 100);
     if (arr[50] != 1) {
         puts("Incorrect memccpy");
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     }
     puts("Correct memccpy");
     int res;
     if ((res = memcmp("hello world", "hello world", 11))) {
         printf("Incorrect memcmp (1), expected 0 found %d\n", res);
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     }
     if ((res = memcmp("hello world", "hello worlt", 11)) >= 0) {
         printf("Incorrect memcmp (2), expected -, found %d\n", res);
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     }
     if ((res = memcmp("hello world", "hallo world", 5)) <= 0) {
         printf("Incorrect memcmp (3), expected +, found %d\n", res);
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     }
     if ((res = memcmp("hello world", "henlo world", 5)) >= 0) {
         printf("Incorrect memcmp (4), expected -, found %d\n", res);
-        return EXIT_FAILURE;
+        exit(EXIT_FAILURE);
     }
     puts("Correct memcmp");
 }

tests/string/strrchr.c

@@ -8,14 +8,14 @@ int main(void) {
   if (ptr != &s0[10]) {
     printf("%p != %p\n", ptr, &s0[10]);
     printf("strrchr FAIL , exit with status code %d\n", 1);
-    return EXIT_FAILURE;
+    exit(EXIT_FAILURE);
   }
   char s1[] = "";
   ptr = strrchr(s1, 'a');
   if (ptr != NULL) {
     printf("%p != 0\n", ptr);
     printf("strrchr FAIL, exit with status code %d\n", 1);
-    return EXIT_FAILURE;
+    exit(EXIT_FAILURE);
   }
   printf("strrch PASS, exiting with status code %d\n", 0);
 }