messages: unbreak /ngx_pagespeed_messages

The shared circular buffer wasn't hooked up fully, which meant loading `/ngx_pagespeed_messages` didn't work.  This fixes that and adds a test.

I also noticed while adding this that the 'Handling of large files' test wasn't set up properly, so I converted that to use start_test.

Fixing that exposed another bug where the 'Handling of large files' test was actually failing but being marked as an expected failure by being grouped in with the test above.  Adding `pagespeed MaxHtmlParseBytes 5000` to the appropriate location made it test what it was supposed to be testing again, and the underlying feature wasn't broken.

Fixes #559