Support running tests in parallel

[RedwoodForest]

It would be nice to be able to run tests in parallel from the command line runner. This would allow us to speed up our continuous builds by reducing test time.

Our tests are written so that they can be run in any order and simultaneously without conflict (so we can run them with, e.g., NCrunch) so we don't require support for more complex cases like requiring sequential execution for specific tests.