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/consent.test.js

@@ -0,0 +1,55 @@
+const helpers = require('./support/helpers');
+
+beforeAll(() => {
+
+  require('../../app/assets/javascripts/govuk/cookie-functions.js');
+  require('../../app/assets/javascripts/consent.js');
+
+});
+
+afterAll(() => {
+
+  require('./support/teardown.js');
+
+});
+
+describe("Cookie consent", () => {
+
+  describe("hasConsentFor", () => {
+
+    afterEach(() => {
+
+      // remove cookie set by tests
+      helpers.deleteCookie('cookies_policy');
+
+    });
+
+    test("If there is no consent cookie, return false", () => {
+
+      expect(window.GOVUK.hasConsentFor('analytics')).toBe(false);
+
+    });
+
+    describe("If a consent cookie is set", () => {
+
+      test("If the category is not saved in the cookie, return false", () => {
+
+        window.GOVUK.setConsentCookie({ 'usage': true });
+
+        expect(window.GOVUK.hasConsentFor('analytics')).toBe(false);
+
+      });
+
+      test("If the category is saved in the cookie, return its value", () => {
+
+        window.GOVUK.setConsentCookie({ 'analytics': true });
+
+        expect(window.GOVUK.hasConsentFor('analytics')).toBe(true);
+
+      });
+
+    });
+
+  });
+
+});