stripecoinpayments.mockStripeClient implements the
stripecoinpayments.StripeClient interface which is used to interact with
stripe's APIs in production. We use it in tests to simulate API calls to
stripe. This change introduces new stripecoinpayments package level
constants: TestPaymentMethodsNewFailure and
TestPaymentMethodsAttachFailure. These can be passed as part of the
arguments their respective stripe mock methods to signal to the method
that it should return an error. This is useful so that we can test how
layers on top of the StripeClient interface handle errors.
Change-Id: Ib64c08ba1e91f31e755b66a1ad563c3b6e77f6f0
