tests/helper-runtests.sh

pylint: split command line argument on multiple lines

This clarify the important line in that configuration.

Differential Revision: https://phab.mercurial-scm.org/D1831

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL