Fix prefetch-purpose layout test to support caching

LayoutTests/http/tests/misc/resources/prefetch-purpose.php

-<?php setcookie("Purpose", $_SERVER["HTTP_PURPOSE"]);
-
-if (isset($_COOKIE['Purpose'])) {
-    setcookie("Purpose", "", time() - 3600);
-    echo "<h1>The cookie was set!</h1>";
-    echo "<p>Purpose: ";
-    echo $_COOKIE['Purpose'];
-} else {
-    echo "<h1>BAD BROWSER NO COOKIE</h1>";
-}
+<?php
+echo "Purpose: " . $_SERVER["HTTP_PURPOSE"];
 ?>
 
 <script>
 testRunner.notifyDone();
 </script>
 
-<p>This test verifies that prefetches are sent with the HTTP request header
-<b>Purpose: prefetch</b>.  To do this, the root page has a prefetch
-link targetting this subresource which contains a PHP script
-(resources/prefetch-purpose.php).  That initial prefetch of this
-resource sets a cookie.  Later, the root page sets window.location to
-target this script, which verifies the presence of the cookie, and
-generates the happy test output that you hopefully see right now.
-
+<p>This test verifies that prefetches are sent with the HTTP request
+header <b>Purpose: prefetch</b>.  To do this, the root page has a
+prefetch link targetting this subresource which contains a PHP script
+(resources/prefetch-purpose.php).  The PHP prints the value of the
+Purpose header into the document.  Later, the root page sets
+window.location to target this script, which should have "Purpose:
+prefetch" in its output if it's served from cache.