constant size for bench.php

i don't like the previous behaviour where the bytes to hash change every time the code change, 
that may make it difficult to compare hash() performance changes over time,

as a recent example, Nikita's commit from 2020-10-26 changed the bytes to hash from 2240 bytes to 503 bytes.. ( e0ea3e8 )

Author: divinity76

ext/hash/bench.php

@@ -3,7 +3,8 @@
 
 error_reporting(E_ALL);
 
-$data = file_get_contents(__FILE__);
+// 2240 comes from the fact that it used to be file_get_contents(__FILE__);
+$data = str_repeat("\x00", 2240);
 $time = array();
 foreach (hash_algos() as $algo) {
     $time[$algo] = 0;