pathutil: add doctests for canonpath()
This is a followup to
f445b10dc7fb. Since there's no way to ensure that more
drive letters than C: exist, this seems like the only way to test it. This is
enough to catch the
f445b10dc7fb scenario, as well as CWD outside of the repo
when the path isn't prefixed with path/to/repo.
#require docutils gettext
$ $TESTDIR/check-gendoc pt_BR
checking for parse errors