Add integration tests for professional ed flow

@dianakhuang This is a first attempt at integration tests for the professional ed flow.  I've focused the tests on three of the changes introduced by professional ed:

1) Skipping the "choose your track page"  (I think there's some overlap here with @flowerhack 's tests in course_modes, but that's okay because this is an integration test.)

2) Showing only one price on the page when there are no suggested prices.

3) Skipping straight to the "verified" page if you've already been verified.

I'm assuming that we already have acceptance-level tests for the full payment flow, which verifies the coordination with SoftwareSecure.  If that hasn't changed significantly from the original verified flow, then I don't think we need additional tests to cover it for prof ed.

Please let me know if there are additional test cases that you think should be included here.
uber nitpick, maybe str(MIN_PRICE)