As Gary said in the comments:
it would be nice if Unit Runner could
be told to go ahead and order them by
class name. Hmm, maybe I should look
into the source code…
I did look but there’s no hint of a functionality to sort these names. I would suggest a change request to the JUnit plugin, but I don’t think, that there are lot of people using this thing, so: DIY.
I would like to see the solution if you modify the plugin code.