Add JS tests for analytics & cookies JS

Includes:
- tests for the analytics interface ported from
  GOVUK Frontend Toolkit
- tests for the cookie banner that appears on all
  pages except the cookies page
- tests for the cookies page JS
- tests for the hasConsentFor function
- adding a deleteCookie helper to remove
  cookies during tests
- polyfill for insertAdjacentText

The last one is because JSDOM doesn't support
insertAdjacentText but our target browsers
do. This polyfill also includes one for
insertAdjacentHTML.

tests/javascripts/analytics/analytics.test.js

@@ -0,0 +1,97 @@
+const helpers = require('../support/helpers');
+
+beforeAll(() => {
+
+  // add the script GA looks for in the document
+  document.body.appendChild(document.createElement('script'));
+
+  require('../../../app/assets/javascripts/govuk/cookie-functions.js');
+  require('../../../app/assets/javascripts/analytics/analytics.js');
+  require('../../../app/assets/javascripts/analytics/init.js');
+
+});
+
+afterAll(() => {
+
+  require('../support/teardown.js');
+
+});
+
+describe("Analytics", () => {
+
+  let analytics;
+
+  beforeEach(() => {
+
+    window.ga = jest.fn();
+
+    analytics = new GOVUK.Analytics({
+      trackingId: 'UA-75215134-1',
+      cookieDomain: 'auto',
+      anonymizeIp: true,
+      displayFeaturesTask: null,
+      transport: 'beacon'
+    });
+
+  });
+
+  afterEach(() => {
+
+    window.ga.mockReset();
+
+  });
+
+  describe("When created", () => {
+
+    test("It configures a tracker", () => {
+
+      setUpArguments = window.ga.mock.calls;
+
+      expect(setUpArguments[0]).toEqual(['create', 'UA-75215134-1', 'auto']);
+      expect(setUpArguments[1]).toEqual(['set', 'anonymizeIp', true]);
+      expect(setUpArguments[2]).toEqual(['set', 'displayFeaturesTask', null]);
+      expect(setUpArguments[3]).toEqual(['set', 'transport', 'beacon']);
+
+    });
+
+  });
+
+  describe("When tracking pageviews", () => {
+
+    test("It sends the right URL for the page if no arguments", () => {
+
+      window.ga.mockClear();
+
+      jest.spyOn(window, 'location', 'get').mockImplementation(() => {
+        return {
+          'pathname': '/privacy',
+          'search': ''
+        };
+      });
+
+      analytics.trackPageview();
+
+      expect(window.ga.mock.calls[0]).toEqual(['send', 'pageview', '/privacy']);
+
+    });
+
+    test("It strips the UUIDs from URLs", () => {
+
+      window.ga.mockClear();
+
+      jest.spyOn(window, 'location', 'get').mockImplementation(() => {
+        return {
+          'pathname': '/services/6658542f-0cad-491f-bec8-ab8457700ead',
+          'search': ''
+        };
+      });
+
+      analytics.trackPageview();
+
+      expect(window.ga.mock.calls[0]).toEqual(['send', 'pageview', '/services/…']);
+
+    });
+
+  });
+
+});