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/helpers/cookies.js

@@ -0,0 +1,22 @@
+// Helper for deleting a cookie
+function deleteCookie (cookieName) {
+
+  document.cookie = cookieName + '=; path=/; expires=' + (new Date());
+
+};
+
+function setCookie (name, value, options) {
+  if (typeof options === 'undefined') {
+    options = {};
+  }
+  var cookieString = name + '=' + value + '; path=/;domain=' + window.location.hostname;
+  if (options.days) {
+    var date = new Date();
+    date.setTime(date.getTime() + (options.days * 24 * 60 * 60 * 1000));
+    cookieString = cookieString + '; expires=' + date.toGMTString();
+  }
+  document.cookie = cookieString;
+};
+
+exports.deleteCookie = deleteCookie;
+exports.setCookie = setCookie;