Tests: make our fixtures/pytest usage less magical

[woodruffw]

We currently do a lot of unnecessary magic in our conftest.py, including nested fixtures and some global modifications using pytest's behavioral hooks. We should identity which pieces of magic we actually need, better encapsulate them, and remove the rest.

In particular, we probably need:

1. A way to parameterize tests as running on both staging and production, with the ability to disable one or the other via a CLI option
2. A way to run tests on only one instance or the other (i.e. staging or production)
3. A better way to mark tests as online and/or needing an ambient OIDC credential