want declarative per-test-class variations

In https://code.edge.launchpad.net/~mbp/bzr/597791-http-tests/+merge/37941 I've tried something in bzr that lets you say this:

  class TestCheckout(TestCase):
    variations = [VaryByRepositoryFormat(), VaryByTreeFormat()]

  load_tests = load_tests_from_their_variations

I would like something like this (perhaps this same code) to be reusable in testscenarios. It builds on scenarios; you can mix and match.

To me this is nice because:

 - The per-class expression is as small as it could reasonably be.
 - There is less magic action-at-a-distance: each class describes how it is varied
 - People are gently encouraged to make reusable classes that describe variations, rather than writing them inline in the load_tests method
 - load_tests, which is a bit of a unittest implementation wart, does not need to be touched by most test code
 - Variations can be composed, given constructor parameters, etc