[bug 937183] Add sanitizing tests

This adds some xss-related tests for sumo.parser.wiki_to_html which we use in a bunch of places. These sorts of issues pop up periodically, so it's easier to have a test for them that we can point to and also to catch possible regressions down the line as we tweak different parts of the parser and upgrade bleach.

We can add to these tests as we go along.

r?