chore(ci): add local HTTP server for Karma tests; (#7022)

test/specs/__helpers.js

@@ -6,6 +6,8 @@ window.axios = _axios;
 jasmine.DEFAULT_TIMEOUT_INTERVAL = 60000;
 jasmine.getEnv().defaultTimeoutInterval = 60000;
 
+window.TEST_SERVER_URL = "http://localhost:3000";
+
 // Get Ajax request using an increasing timeout to retry
 window.getAjaxRequest = (function () {
   let attempts = 0;

test/specs/formdata.spec.js

@@ -3,7 +3,7 @@ import {retryNetwork} from "../helpers/retry.js";
 describe('FormData', function() {
   it('should allow FormData posting', async () => {
     await retryNetwork(() => {
-      return axios.postForm('http://httpbin.org/post', {
+      return axios.postForm(TEST_SERVER_URL, {
         a: 'foo',
         b: 'bar'
       }).then(({data}) => {