I think the idea that only people intending to do development work on MediaWiki download from SVN is a bit insane. And as you note, these tests are only going to grow in size over time.
This whole discussion is bringing an analogy to a PHP framework that I'm using a lot now, symfony [1]. Their SVN is even larger than MediaWiki's. A large part of that is for languages (which, by the way, are not only languages, but every single culture too), and lots of unit tests. 99.99% of users don't need them, but they package them anyway. I think that the whole discussion should be moot, as 40MB is not that much in comparison to other codebases. Either way, people will need far more than 40MB of space on the server to run a good wiki, and not very many people have a server that would have less in the first place.
-X!