The block gets `class_eval`'d. I haven't found a good use for this one,
exactly, but it's good to know about!
-EDIT: @pete_higgins has chimed in on Twitter. I knew I'd done this before:
-<blockquote class="twitter-tweet" data-in-reply-to="244902236185047041"><p>@<a href="https://twitter.com/steveklabnik">steveklabnik</a> Class.new with a block is useful for creating anonymous classes in tests, or as a simple mock mechanism.</p>— pete higgins (@pete_higgins) <a href="https://twitter.com/pete_higgins/status/244903490588119040" data-datetime="2012-09-09T21:01:56+00:00">September 9, 2012</a></blockquote>