Instead of using python directly in a subprocess, try calling a shell command that in turns calls python.  (Admittedly, this may look like the pipe scenario...)

In theory, you could even drive another python interactively, using a GUI runner, but I'm not sure how much new infrastructure that would add to the test suite, so it might not be worth it.

I'm not aware of a list of manual tests (though perhaps there *should* be one...); I would put it under the DeprecationWarning docs, sort of like an example.  Others may have better ideas.