tests: avoid implicit conversion of str to unicode
On Python 2, str.encode('ascii') implicitly converts the string to unicode and
then back to str.
Since the converted value is a path, _bytespath can be used instead.
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif