Run tests as authenticated user

We should figure a way to keep the user session active after logging in to the application. Currently the session does not persist across multiple app.test_client() invocations, and as such, we're unable to properly test endpoints which require authentication or have different behavior based on the user role (admin, user, sponsor).

Once this has been sorted out, the code which sets the app.config["TESTING"] parameter to True should be deleted.