Expose some of the Auth test utility functions.

[blink1073]

Problem

We had to inline classes, fixtures, and functions in https://github.com/jupyter-server/jupyter_server_terminals/pull/11 that should probably be exposed here so other extensions can use them to test auth on their extension.

Proposed Solution

Move the pieces that are used in https://github.com/jupyter-server/jupyter_server_terminals/pull/11 and in tests/auth/authorizer.py into jupyter_server.pytest_plugins.py