python: Pass $PYTHON environment variable to tests.

python: Pass $PYTHON environment variable to tests.

If the user set PYTHON when configuring, this variable is not passed
through to the tests, so it is possible the tests will fail because
they are testing the wrong version of python.  By passing $PYTHON
through to the tests we ensure that we test against the same version
of python that we configured with.
(cherry picked from commit e2249b7ce1dd0a2f8f110e0e47aca397185a6373)