Rename tracker in tests

tracker is an object that comes directly from google. This is why the `get`
method call has been kept on it, it's not a jQ object so the method is
defined by google rather than jQ. I've renamed the `tracker` object in the
tests to make it more obvious that it's something we don't have ownership
over

spec/javascripts/set-ga-client-id-on-form.spec.js

@@ -2,9 +2,9 @@ var $ = window.jQuery
 
 describe('SetGaClientIdOnForm', function () {
   var GOVUK = window.GOVUK
-  var tracker = { clientId: 'clientId' }
-  tracker.get = function (arg) { return this[arg] }
-  window.ga = function (callback) { callback(tracker) }
+  var mockTracker = { clientId: 'clientId' }
+  mockTracker.get = function (arg) { return this[arg] }
+  window.ga = function (callback) { callback(mockTracker) }
   var form
 
   beforeEach(function () {