Tests: IE8 support for log capture

No Array.indexOf. We are living like animals!
Tests: update AJAX tests

* IE8 support for makeAjaxRequest test (xhr.response does not exist,
  responseText is supported by all major browsers)
* Update makeAjaxRequest test to confirm that the success callback is
  called and that the error callback is not
* Add a test for makeAjaxRequest with an invalid file and verify that
  error callback is called but the success callback is not