DEV: add frontend tests to make sure pricing-table works

We test three conditions:
1. Make sure the customer email is prefilled, if there is a logged in discourse user
2. The pricing table also displays, if there is no logged in discourse user
3. If the pricing table is not configured, the pricing table element does not show up.

test/javascripts/helpers/product-pretender.js

@@ -1,7 +1,7 @@
 export default function (helpers) {
   const { response } = helpers;
 
-  this.get("/s", () => {
+  this.get("/subscriptions", () => {
     const products = [
       {
         id: "prod_23o8I7tU4g56",
@@ -20,7 +20,7 @@ export default function (helpers) {
     return response(products);
   });
 
-  this.get("/s/:id", () => {
+  this.get("/subscriptions/:id", () => {
     const product = {
       id: "prod_23o8I7tU4g56",
       name: "Awesome Product",