Isolate tests from environment variables

> As it is, tests don’t use a fixture for the configuration object, so it can be influenced by environment variables. The regular Python library tests will also fail when run with `ANTHOLOGY_PREFIX="..." just test-all`. We should either:
> 1. Change tests to use a fixed configuration object that’s independent of environment variables.
> 2. Just accept as a convention that tests should never be called with a different `ANTHOLOGY_PREFIX`.

_Originally posted by @mbollmann in https://github.com/acl-org/acl-anthology/issues/9882#issuecomment-5560703324_