Fix SubscriptionConfirmJob spec

Fix the payment stubbing, payment `process_offline!` was stubbed to
return true and payment `completed?` was also stubbed to return true.
`process_offline!` did not change the payment status in the database,
meaning that any call to 'order.pending_payments' after that would
return the payment which should have been processed offline, and
therefore completed. This created some unwanted side effect, resulting
in the test breaking.

spec/jobs/subscription_confirm_job_spec.rb

@@ -239,8 +239,11 @@ describe SubscriptionConfirmJob do
 
         context "when payments are processed without error" do
           before do
-            expect(payment).to receive(:process_offline!) { true }
-            expect(payment).to receive(:completed?) { true }
+            allow(payment).to receive(:process_offline!) do
+              # Mark payment as complete like it would be if sucessfully processed offline
+              payment.state = "complete"
+              true
+            end
           end
 
           it "sends only a subscription confirm email, no regular confirmation emails" do