test: add a function to run Python tests

The new test_python() function makes writing Python tests a little easier:
- it sets the environment variables as needed
- it redirects stdout to the OUTPUT file (like test_emacs()).

This commit also declares python as an external prereq.

The stdout redirection is required to avoid trouble when running commands like
"python 'script' | sort > OUTPUT": in such a case, any error due to a missing
external prereq would be "swallowed" by sort, resulting to a failed test instead
of a skipped one.