Add supporting classes for test helper generation

[pakrym-stripe]

Test helper methods on a resource are placed inside a nested TestHelpers class:

        module Stripe
          class Foo < APIResource
            extend Stripe::APIOperations::Create

            OBJECT_NAME = 'foo'

            custom_method :pdf, http_verb: :get

            def pdf(params = {}, opts = {})
              request_stripe_object(
                method: :get,
                path: resource_url + '/pdf',
                params: params,
                opts: opts,
              )
            end

            def test_helpers
              TestHelpers.new(self)
            end

            class TestHelpers < APIResourceTestHelpers
              RESOURCE_CLASS = Foo

              custom_method :boop, http_verb: :post

              def boop(params = {}, opts = {})
                @resource.request_stripe_object(
                    method: :post,
                    path: resource_url + '/boop',
                    params: params,
                    opts: opts,
                  )
              end
            end
          end
        end

To support the generation we need:

1. Add a base APIResourceTestHelpers with test-helper URL generation logic.
2. Adjust custom_method to handle cases where static method in not on the resource class itself.

[richardm-stripe]

CI is failing, one more file needs to be pushed?

[richardm-stripe]

This looks good, let's wait to merge until the first test_helper method is released.

[dcr-stripe]

Looks good, just one nit and then we'll be good to ship! Thanks!

[pakrym-stripe]

PTAL @dcr-stripe ?

[dcr-stripe]

LGTM!