Apply suggestions from code review

Co-authored-by: Ilija Tovilo <[email protected]>

Author: nielsdos

tests/output/gh15179.phpt

@@ -10,12 +10,9 @@ if (getenv("USE_ZEND_ALLOC") === "0") die("skip requires ZendMM");
 ?>
 --FILE--
 <?php
-$var = '';
-for ($i = 0; $i < 20; $i++) {
-    $var .= str_repeat('a', 1 * 1024 * 1024);
-}
+$var = str_repeat('a', 20 * 1024 * 1024);
 
 output_add_rewrite_var($var, $var);
 ?>
 --EXPECTF--
-Fatal error: Allowed memory size of 67108864 bytes exhausted at %s
+Fatal error: Allowed memory size of %d bytes exhausted at %s