Refactor default_store logic in paver test_bokchoy

@clytwynec @nasthagiri 

Hey guys I refactored this. In part, it makes it easier to test. But, I still need to sort out mocking the run_bokchoy wherein I can examine the test suite object but avoid actually running the script.

That said, could you please take a look at the 'production' code as well. Any feedback?

Thanks!
why is the default `None` here?