FileWriter output = new FileWriter("/tmp/testlog");
StreamResult result = new StreamResult(output);

The resulting output has garbled UTF-8 characters.

I am under the impression that java.io.FileWrite will
change the character encoding to "the native encoding
of the platform".

Simply use java.io.File instead. This allows the Saxon
serializer to do the encoding itself and write the
resulting bytes directly into the file.