Refactor TestRender and TestStripeErrors

 And Add CommonFixture parent class

 These test cases test different kinds of things

TestRender: (Incomplete)
 tests that the processing within SubmitSubcriptionHandler.render
 In particular these tests (when complete) will patch out calls
 to other methods of SubmitSubcriptionHandler.  This will mean
 that test failures can generally be understood to be method specific
 within the context of SubmitSubscriptionHandler methods.

 TODO:
  test that variables e.g. stripe_api_key, subscriptions_fp, etc. have
  correct assignments and are passed to appropriate functions
 TODO:
  test that appropriate/expected mocks are called

TestStripeErrors: (Incomplete)
 tests that the SubmitSubscriptionHandler deals with each
 error message it can receive from stripe, appropriately.
 In particular these tests never invoke the
 SubmitSubcriptionHandler.render method.

 This suite currently tests the exception cases within '.create_customer'.
 IIUC .create_customer is sufficiently tested.
 TODO:
  test the logic within '.handle_stripe_create_customer_errors'.

TODO:
 Add TestCases for other methods:
  .__init__, .get_stripe_api_key, .get_creation_parameters, and .run_full_signup

TODO:
 Add TestCase for start.

NOTE:  class CommonFixture(TestCase) has no tests of its own