Add some breaking tests for summary generation via markdown.

When I allow a summary to be generated using a summary separator on a markdown enabled article I end up missing a `</p>` tag. It looks like the paragraph is being closed after the summary separator (by markdown) and then, when the rendered content is split in order to create the summary, the closing paragraph is lost in the discarded section.

I thought of splitting the content before rendering and then applying the markdown to the content before the summary separator but I came across the problems outlined in issue #80. Right now I have no solution for this, so I've merely tweaked the tests to report on the problem.