Generally, the results files for fn-unordered-mix-args seem to assume that
unordered() is a no-op - which is likely to be true on many implementations but
not all.

However, any serialization of the result of 

fn:unordered ( ("a", xs:string(""), "b", "c")) 

must be exactly six characters long: 1 each for "a", "b", and "c"; and three for
the single-space separators. In this case, if unordered() is a no-op, the result
should be "a  b c" not "a b c".

The result file, like many others, also contains a trailing newline which should
not be there.