Refactor to avoid needing to patch `sys.argv`

[ajm188]

As per discussion here

[evhub]

Not a big deal, but if you make this change the testing becomes slightly less end-to-end, since if there's a mistake telling argparse to grab sys.argv, that would never be caught.

[ajm188]

True.

However, I would argue that a test that uses a mock isn't really an end to end test, since it wouldn't catch if there was some code that messed with sys.argv before argparse got its hands on it (which is basically what mock does).

Of course, this would never happen in practice, but I doubt we would not catch a change that told argparse to grab something other than sys.argv in production.