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/polyfills.js

@@ -0,0 +1,53 @@
+// Polyfills for any parts of the DOM API available in browsers but not JSDOM
+
+// From: https://gist.github.com/eligrey/1276030
+HTMLElement.prototype.insertAdjacentHTML = function(position, html) {
+  "use strict";
+
+  var
+      ref = this
+    , container = ref.ownerDocument.createElementNS("http://www.w3.org/1999/xhtml", "_")
+    , ref_parent = ref.parentNode
+    , node, first_child, next_sibling
+  ;
+
+  container.innerHTML = html;
+
+  switch (position.toLowerCase()) {
+    case "beforebegin":
+      while ((node = container.firstChild)) {
+        ref_parent.insertBefore(node, ref);
+      }
+      break;
+    case "afterbegin":
+      first_child = ref.firstChild;
+      while ((node = container.lastChild)) {
+        first_child = ref.insertBefore(node, first_child);
+      }
+      break;
+    case "beforeend":
+      while ((node = container.firstChild)) {
+        ref.appendChild(node);
+      }
+      break;
+    case "afterend":
+      next_sibling = ref.nextSibling;
+      while ((node = container.lastChild)) {
+        next_sibling = ref_parent.insertBefore(node, next_sibling);
+      }
+      break;
+  }
+
+};
+
+// from: https://developer.mozilla.org/en-US/docs/Web/API/Element/insertAdjacentText#Polyfill
+if (!Element.prototype.insertAdjacentText) {
+  Element.prototype.insertAdjacentText = function(type, txt){
+    this.insertAdjacentHTML(
+      type,
+      (txt+'') // convert to string
+        .replace(/&/g, '&') // embed ampersand symbols
+        .replace(/</g, '&lt;') // embed less-than symbols
+    )
+  }
+}