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

tests/test-check-pyflakes.t

@@ -21,4 +21,5 b' run pyflakes on all tracked files ending'
   > -X mercurial/thirdparty/zope \
   > 2>/dev/null \
   > | xargs pyflakes 2>/dev/null | "$TESTDIR/filterpyflakes.py"
+  contrib/perf.py:*: undefined name 'xrange' (glob) (?)