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/support/setup.js

@@ -1,3 +1,6 @@
+// Polyfill holes in JSDOM
+require('./polyfills.js');
+
 // set up jQuery
 window.jQuery = require('jquery');
 $ = window.jQuery;