A `hub_app` fixture  that depends on `jupyterhub_config`

### Proposed change

A `hub_app` fixture  that depends on `jupyterhub_config` and the existing and returns a started JupyterHub instance, so folks don't need to write the fixture to start the app every time

### Who would use this feature?

Spawners, authenticators

Reference #65 by @minrk 