Update testing chapter structure and add detail

[foster999]

No description provided.

[foster999]

Need to improve guidance on integration testing. Should be above unit test level (i.e. not focused on values that are created by individual functions). Better to look at schema of data or properties of data (e.g. missingness)

Discuss trade-off when integration tests go into more detail (regression test), making test fragile

Need to discuss acceptance tests and how these should be automated
Need to change the wording of the checklist criteria to something like "It's easy to identify which UATs have passed against a given change in the code version control".
Need guidance for migrating from manual testing to documented outcomes near the code, to automated tests
"Near code" means that we can get from code to the tests that have passed (the acceptance criteria)

When testing exceptions, test the specific exception type and error message if used

[gisellerosetta]

This is great, really thorough - have added a comment onto #87 so the additional work to do on this isn't lost as we keep developing the chapter.

Many thanks @foster999 !