In order to facilitate testing, but not necessarily as an example of
good practice, I am changing the invocation of the test tools to use
`/usr/bin/env python`, so that we can control which python (2 vs 3)
gets invoked for the test. On the buildbots, I plan to add an optional
`ln -s /usr/bin/python3 /usr/local/bin/python` on systems that have
python3-bcc package built. This way, we get more test coverage. Having a
cmake mechanism to enable both python2 and python3 testing could be a
further enhancement.