Add tests for some more differing URI scenarios.

[Julian]

Covers:

* A $ref to an absolute URI present in the retrieved schema
* A schema with URN $id with a nested $ref
* Retrieving a schema with a different $id than the retrieval URI
* Retrieving a schema with $ref whose retrieval URI was an HTTP URI
  but whose $id is an URN

(These involve sibling $refs so don't apply to pre-2019).

Missing is a test for an URN retrieval URL, which unfortunately we have
no way of communicating at the minute.

Refs: #578 (comment)

[gregsdennis]

I'll let others speak to the correctness of these tests (I'm not good with URIs), but these tests all pass on my implementation.

[karenetheridge]

they pass here too.

[Julian]

Thanks both.

[Julian]

Sorry, I guess I see @karenetheridge you didn't comment on the tests themselves either... Can you speak up if you (or anyone) has comments on the tests (I suppose now which I can address in a follow up)