tests: invoke run-tests.py in test-hghave using $PYTHON (
issue5697)
Some platforms (notably pkgsrc on NetBSD) only provide versioned
Python interpreters (eg `python2.7` exists, but `python` does not),
which exposes this error. We want to be running run-tests.py with the
python given in $PYTHON, since that might be `pypy` or `python3`.