Add backend flag for pytest runs

Adds a backend flag to pytest so as we add backends we can target a single backend for testing without having to do collection/skipping on the other backends. Also adds {posargs} to tox so we can pass it through to tox.

This does require adding a new attribute (name) to a given backend. If others know a more elegant way to do this please let me know; this is just what I came up with given my experience level with pytest.