Harness can include an IdP to self-issue tokens

[elf-pavlik]

I'm thinking about approach similar to https://github.com/solidcouch/solid-identity
With the difference that test client could issue its own tokens including webid and client_id / azp claims. It would require adding the IdP oidcIssuer reference to test users' WebID profiles but that seems simpler than dealing with client credentials, refresh tokens etc.