tests: allow warning about file
The code is protected by a block that makes it Python 2 only, so
use of file is acceptable.
Differential Revision: https://phab.mercurial-scm.org/D7018
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state