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

@@ -1,24 +1,25 b''
 #require test-repo pyflakes hg10
 
   $ . "$TESTDIR/helpers-testrepo.sh"
 
 run pyflakes on all tracked files ending in .py or without a file ending
 (skipping binary file random-seed)
 
   $ cat > test.py <<EOF
   > print(undefinedname)
   > EOF
   $ pyflakes test.py 2>/dev/null | "$TESTDIR/filterpyflakes.py"
   test.py:1: undefined name 'undefinedname'
   
   $ cd "`dirname "$TESTDIR"`"
 
   $ testrepohg locate 'set:**.py or grep("^#!.*python")' \
   > -X hgext/fsmonitor/pywatchman \
   > -X mercurial/pycompat.py -X contrib/python-zstandard \
   > -X mercurial/thirdparty/cbor \
   > -X mercurial/thirdparty/concurrent \
   > -X mercurial/thirdparty/zope \
   > 2>/dev/null \
   > | xargs pyflakes 2>/dev/null | "$TESTDIR/filterpyflakes.py"
+  contrib/perf.py:*: undefined name 'xrange' (glob) (?)