Fix broken test in IE11.

It's not clear why this fixes the test; the actual error is known and not
fixed/understood: chaijs/sinon-chai#64

Author: markrian

test/mouse-events.js

@@ -2,6 +2,7 @@
 
 describe('Mouse Events', function () {
   var fixture = document.createElement('div');
+  document.body.appendChild(fixture);
 
   describe('click', function () {
     it('should trigger', function () {