HTTP Client 2.0 tests

[adamretter]

We need a test suite for HTTP Client 2.0.

For example, at the moment I want to make sure we can handle the various XDM <-> HTTP JSON request/responses that we want, e.g. sending null etc.

I think we should specify it using the same grammar as https://github.com/w3c/qt3tests. AFAIK the following processors already support that:

1. BaseX
2. eXist-db
3. Saxon

We should be able to easily add something to the environment of the qt3tests grammar to define/mock a simple HTTP response from the server. I would be happy to take on this task if others are in agreement, and will contribute tests too.