tests/helper-runtests.sh

cleanup: use $PYTHON to run python in many more tests

Spotted one of these, then wrote a check-code rule that caught them
all. It will be the next change.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL