Add a failing test for a URL helper that was broken by a6b9ea2.

Between Rails 4.0.3 and 4.0.4 (and into 4.1.0), the routes in this test stopped producing the URL helper `comment_path` -- it became `blog_comment_path` instead. This was fixed in a6b9ea2 but the patch breaks the expected `new_blog_post_comment_path` helper, which was working until that patch landed.