Use a mock IdP

[edwardsph]

The tests are currently dependant on inrupt.net or solidcommunity.net as the IdP - it would be better to have a mock IdP as part of the test harness.

[kjetilk]

We had https://github.com/solid/test-idp as I found it convenient, especially given my use of relative URIs for the system under test, to have an IdP running that would simply supply tokens to anyone that asked, and also supply invalid tokens for the purpose of shooting the server in the foot.

It is clearly out of date with respect to Solid-OIDC, but perhaps there's something that could be pillaged.