CTest filters the output from tools and tests to ensure that the XML
build/test result documents it generates have valid characters.
Previously we just converted all non-ASCII bytes into XML-escaped
Unicode characters of the corresponding index.  This does not preserve
tool output encoded in UTF-8.

We now assume UTF-8 output from tools and implement decoding as
specified in RFC 3629.  Valid characters are preserved, possibly with
XML escaping.  Invalid byte sequences and characters are converted to
human-readable hex values with distinguishing tags.  See issue #10003.