WIP - Update transaction and subscription integration tests

Add instance variables for the created payment methods to
make them easier to track in tests.

Update subscriptions integration test to work with the
payment method select field.
Generate PurchasePurpose scaffold
Add passing model tests for PurchasePurpose

Add relationship tests.

Add a test to make sure the proper user_id is assigned when saved.

Add a test to validate the presence of purpose.

Add a test to make sure the purpose is unique within the scope of
the user.
Add a uniqueness test to a PaymentMethod's name scoped to user_id
WIP - Start writing PurchasePurpose integration tests