This test was failing in some Python 3 environments because
$PYTHON -m virtualenv was somehow resulting in Python 2
being used. Why, I'm not sure.
Python 3 includes virtualenv in the standard library as the
venv module.
This commit changes test-install.t to use $PYTHON -m venv on
Python 3 and $PYTHON -m virtualenv on Python 2 (if available).
I chose to make some test output duplicated because we can't
have nested conditionals and there is no easy way to express
ORing of hghave checks.