[Test] Test reorganisation

[riedgar-ms]

Start reorganising the tests into more directories. The major divisions introduced are:

• unit for tests which at most require a Mock model
• model_integration for tests which require a 'real' model, but should work with any model
• model_specific for isolating issues we've found with particular LLMs
• need_credentials .... exactly what it says. Mainly remote-endpoint tests which don't have full Guidance support
• notebooks again, exactly what it says
• server contains tests of the local server. Technically unit tests, but run slowly

The workflows have been updated to match.

[riedgar-ms]

This is just a beginning; I've not started redoing the workflow files yet.

[riedgar-ms]

Also note that some files (e.g. test_model.py and test_grammar.py have been split into two between unit and model_integration according to whether each test needs a real model or not.

[hudson-ai]

@riedgar-ms thanks for the work on this. I think the logical separation between types of tests makes sense, and upon thinking about it a bit more, I totally agree with the decision to use folders rather than pytest.mark. And I'm digging the fast unit tests :)