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!
I was thinking we leave this as None because we're overriding it anyway (but we need to set a default in the object). It is part clarity and it is partly leaving room for future refactoring. I'm not strictly tied to it though.