Add a test which `cp`s `__main__.py` as `duct` and executes basic test/run

[yarikoptic]

While traveling I recalled why I wanted to keep duct (but not necessarily duct-suite) in a single file: to ease deployment/use as part of larger "runners on remote location". E.g. to replace "built-in" functionality within reproman from which we took the idea of capturing stdout/stderr. For that we need a basic Python (no depends) script easy to scp and run.

For the same reason it should get shebang !#/usr/bin/env python3