tests/helper-runtests.sh

heptapod-ci: run test with python3 too

Python3 is the future^W present, it is important to run tests with it too.

Differential Revision: https://phab.mercurial-scm.org/D8013

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL