MDC unit testing

I'm collecting ideas here for building a unit testing quite for MDC. This is really rough so far, with lots left to add!

Editor

• Test that the definition list plugin is installed and working.
• Test to be sure that the "mdc" plugin, which hides and shows the sidebar while editing, is working.

General page content extensions

• Make sure that the IRC links appear in the sidebar, and that they go to the right IRC rooms.

Admin area tests

• Make sure the TagRename special page works.

Multi-host tests

• Make sure that repeated attempts to visit pages work.
• Make sure that MindTouch extensions work by visiting pages that use them repeatedly; sometimes they fail to start up on one or both hosts.