xunit: don't claim to take multiple args in Tee.write()
Fix #680: xunit problem with unicode on stdout

It turns out we were trying to write unicode strings into a
cStringIO.StringIO object, which doesn't work well with unicode strings.
Instead, lets use StringIO.StringIO objects for capturing stdout and
stderr.
Merge pull request #684 from jszakmeister/fix-xunit-unicode

Fix xunit when writing unicode to stdout and stderr.