Refactor nsjail.py for cleaner testing

By factoring out the system calls from the
pure functions we can now:

* Test the core functionality independently from
  any system dependencies.
* Add a regression test for a recent high impact bug
  that was masking build failures.
* Remove all reliance on testing mocks.

Change-Id: Ia9d5686b5e6cc73275d308b3bb35ef1408f8fa35