Documentation for updating the unit tests

[mkovari]

One of the factors slowing down development is that at least one unit test always has to be updated after any change that alters the output. It would help if the procedure for updating the tests was documented.

@timothy-nunn @chris-ashe @clmould ?

[timothy-nunn]

The scope of this issue is documenting how to write a unit test or update an existing test. I believe this can be achieved by updating the existing unit test description to provide a deeper definition of a unit test. Linking to pytest documentation will provide most of the understanding for writing a unit test.

Within our docs, I suggest we just include a light description of:

• How to provide models via fixtures (where the code to test is a model method)
• The basic anatomy of a unit test (provide a reproducible environment, call the function with realistic data, make assertions about the return data/state)
• A few best-practices (e.g. test branches and intentional failure modes)