tests: use venv on Python 3...
tests: use venv on Python 3
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.
Differential Revision:
https://phab.mercurial-scm.org/D7224
