I've always believed that a language should have as little native as possible; the more you can off-load into a library, the better because it lets people extend the language in the language.

Why have first-class unit tests when they could be built on top of some other feature? If tests are part of the language, then using a different testing system (say something in the style of QuickCheck) would probably be at a disadvantage; if you have a language that can support expressive testing as a library, then it would be possible to add different styles of testing.