Fix prefetch-purpose layout test to support caching

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

 prefetch-purpose.php has MIME type text/html
 prefetch-purpose.php has MIME type text/html
-The cookie was set!
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 Purpose: prefetch
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-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.