Add missing method to StripeJS mock

Our Angular code calls this method for interacting with live form validations and messages. We don't really need to use in tests, it just needs to exist otherwise the specs fail.

spec/support/fixtures/stripejs-mock.js

@@ -10,6 +10,10 @@ class Element {
       <input name="cvc" placeholder="CVC" size="3" type="text">
     `;
   }
+
+  addEventListener(event) {
+    return true;
+  }
 }
 
 window.Stripe = () => {