tests: allow xrange warning from perf.py
The use of xrange in this file is acceptable.
We need to make the output optional - and not conditional on
the Python version - because `pyflakes` could be executed by any
Python version and pyflakes behaves differently depending on the
Python version.
Differential Revision: https://phab.mercurial-scm.org/D6990
#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