This tests the XMLHttpRequest responseXML loading an XML document with responseType "document".
On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
PASS xhr.responseXML is non-null.
PASS xhr.responseXML instanceof Document is true
PASS id3 is non-null.
PASS id3.textContent is "Three"
PASS successfullyParsed is true
TEST COMPLETE