[Tests] Change desired XHR response type

Don't request json as an XHR response type, as this
is not supported by Phantom for command line test
execution. Part of ongoing intitial repository and
build setup for WTD-519.

test.html

@@ -48,10 +48,9 @@
                 function loadJSON(file, callback) {
                     var xhr = new XMLHttpRequest();
                     xhr.open("GET", file);
-                    xhr.responseType = "json";
                     xhr.onreadystatechange = function () {
                         if (xhr.readyState === 4) { // DONE
-                            callback(xhr.response);
+                            callback(JSON.parse(xhr.response));
                         }
                     }
                     xhr.send();