tests/helper-runtests.sh

py3: convert os.devnull to bytes using pycompat.bytestr

os.devnull returns str on Python 3.

Differential Revision: https://phab.mercurial-scm.org/D2477

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL