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/init.test.js

@@ -0,0 +1,123 @@
+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 init", () => {
+
+  beforeAll(() => {
+
+    window.ga = jest.fn();
+    jest.spyOn(window.GOVUK.Analytics, 'load');
+
+    // pretend we're on the /privacy page
+    jest.spyOn(window, 'location', 'get').mockImplementation(() => {
+      return {
+        'pathname': '/privacy',
+        'search': ''
+      };
+    });
+
+  });
+
+  afterEach(() => {
+
+    window.GOVUK.Analytics.load.mockClear();
+    window.ga.mockClear();
+
+  });
+
+  test("After the init.js script has been loaded, Google Analytics will be disabled", () => {
+
+    expect(window['ga-disable-UA-26179049-1']).toBe(true);
+
+  });
+
+  describe("If initAnalytics has already been called", () => {
+
+    beforeAll(() => {
+
+      // Fake a tracker instance
+      window.GOVUK.analytics = {};
+
+    });
+
+    beforeEach(() => {
+
+      window.GOVUK.initAnalytics();
+
+    });
+
+    afterAll(() => {
+
+      delete window.GOVUK.analytics;
+
+    });
+
+    test("The Google Analytics libraries will not be loaded", () => {
+
+      expect(window.GOVUK.Analytics.load).not.toHaveBeenCalled();
+
+    });
+
+  });
+
+  describe("If initAnalytics has not been called", () => {
+
+    beforeEach(() => {
+
+      window.GOVUK.initAnalytics();
+
+    });
+
+    afterEach(() => {
+
+      // window.GOVUK.initAnalytics sets up a new window.GOVUK.analytics which needs clearing
+      delete window.GOVUK.analytics;
+
+    });
+
+    test("Google Analytics will not be disabled", () => {
+
+      expect(window['ga-disable-UA-26179049-1']).toBe(false);
+
+    });
+
+    test("The Google Analytics libraries will have been loaded", () => {
+
+      expect(window.GOVUK.Analytics.load).toHaveBeenCalled();
+
+    });
+
+    test("There will be an interface with the Google Analytics API", () => {
+
+      expect(window.GOVUK.analytics).toBeDefined();
+
+    });
+
+    test("A pageview will be registered", () => {
+
+      expect(window.ga.mock.calls.length).toEqual(5);
+
+      // The first 4 calls configure the analytics tracker. All subsequent calls send data
+      expect(window.ga.mock.calls[4]).toEqual(['send', 'pageview', '/privacy']);
+
+    });
+
+  });
+
+});