Refactor unit tests to better use pytest functionality (e.g. fixtures)

[Tom-Kingstone]

Definition of the test :

see the docs: https://docs.pytest.org/en/latest/how-to/fixtures.html

The main reason to do this is so that tests don't actually affect each other as I have been discovering while trying to write a test for #318, where the setup of an external comfort object in a certain way is the actual test