Allow PhortuneTestProvider to add payment methods

Summary: Provide a bare implementation so that you can add PhortuneTestProvider as a payment method. Ref 2787.

Test Plan: Added "cards" through the test provider.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D5772