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.
Merge pull request #560 from pagespeed/jefftk-fix-messages

messages: unbreak /ngx_pagespeed_messages