Add tests for test experiment setup

[jo-basevi]

In the tests for repro tests (e.g. tests/test_test_bit_reproducibility.py), there's some checks for the test setup which can be separated out (as suggested in this PR). There probably should be some unit tests for exp_test_helper.py checking the setup methods in a separate test file. Similarly for each model's setup - e.g. set_model_runtime.

[anton-seaice]

This is where talking about tests of tests gets very confusing 🫤

test_test_bit_repro_historical_pass is testing two things - one is that historical repro tests pass when the should pass and the other is that model-config-tests has setup the model correctly for the test. It would be clearer / easier to use if this was split into two tests so that failures are clear.

[anton-seaice]

Whilst we are looking at this file - the use of the term mock is incorrect for the test setup done.

A mock is a test object which enforces or checks the behavior of the code under test.

We have used the function create_mock_output here to create some expected output - this should just be referred to as dummy or fake output

See https://martinfowler.com/articles/mocksArentStubs.html

[jo-basevi]

Thanks, that's good to know!