Python tests are non-hermetic

The python launcher uses `exec.LookPath` to find `python` in the `PATH`.  As a result, tests fail on vanilla'ish macOS 12.3 where Python 2.7 was removed and Python 3 is only available as `python3`.

Since the tests don't actually launch Python, we should have the lookup be pluggable.

Workaround: create a `python` in the PATH before running the tests.