According to https://www.w3.org/TR/2017/REC-xslt-30-20170608/#splitting,
"The value of the media-type attribute provides the value of the media-type parameter to the serialization method. The default value is text/xml in the case of the xml output method, text/html in the case of the html and xhtml output methods, and text/plain in the case of the text output method. "

The tests sets the serialization method to 'html' and expects the media-type to be text/html, but encounters 'text/xml' because of which the test fails.