Fix prefetch-purpose layout test to support caching

LayoutTests/http/tests/misc/prefetch-purpose-expected.txt

Index: LayoutTests/http/tests/misc/prefetch-purpose-expected.txt
diff --git a/LayoutTests/http/tests/misc/prefetch-purpose-expected.txt b/LayoutTests/http/tests/misc/prefetch-purpose-expected.txt
index 52acec5c8882769a8a0ac0bf68b6e6930e9df2a8..dc582e7d7bc53671ce4ac63e7272e85c7fdcfe30 100644
--- a/LayoutTests/http/tests/misc/prefetch-purpose-expected.txt
+++ b/LayoutTests/http/tests/misc/prefetch-purpose-expected.txt
@@ -1,7 +1,4 @@
 prefetch-purpose.php has MIME type text/html
 prefetch-purpose.php has MIME type text/html
-The cookie was set!
-
 Purpose: prefetch
-
-This test verifies that prefetches are sent with the HTTP request header Purpose: prefetch. 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.
+This test verifies that prefetches are sent with the HTTP request header Purpose: prefetch. 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.