Rewrite cmdline_test.cpp in Python

Summary:
This test tends to flake out.  Having it in Python, spawning new subprocesses
each time, will likely be more stable.  Plus setting the command
line is way easier outside of the native extension.

Reviewed By: yoney

Differential Revision: D114447243

fbshipit-source-id: 7b23016c9ebf5a8daef44ef164ef8ac09126294f

Author: alexmalyshev