Preparatory refactoring of the OidcHandlerTestCase (#8911)

* Remove references to handler._auth_handler

(and replace them with hs.get_auth_handler)

* Factor out a utility function for building Requests

* Remove mocks of `OidcHandler._map_userinfo_to_user`

This method is going away, so mocking it out is no longer a valid approach.

Instead, we mock out lower-level methods (eg _remote_id_from_userinfo), or
simply allow the regular implementation to proceed and update the expectations
accordingly.

* Remove references to `OidcHandler._map_userinfo_to_user` from tests

This method is going away, so we can no longer use it as a test point. Instead
we build mock "callback" requests which we pass into `handle_oidc_callback`,
and verify correct behaviour by mocking out `AuthHandler.complete_sso_login`.