tests: fix error case in test-url.py's doctest
This required some careful attention, so I wanted to split it out from
the uninteresting bits that'll be in the next change.
Differential Revision: https://phab.mercurial-scm.org/D3469
#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