Clean up handling of argv in tests

https://github.com/Kinto/kinto-wizard/blob/762c573a4d405b7a19a1d796992d45cd80f560d7/tests/test_functional.py#L23-L26

This sort of stuff seems a little bit like bad practice..

Start in tests/test_functional.py at lines 23-26 and inspect how argv is handled in the surrounding tests. Run the relevant functional test suite to establish current behavior, then define a less problematic test setup while preserving the existing test results; done means the argv handling is cleaned up without regressions.

Written by the indexing model from the issue text.