Improvements to testing

**Which module(s) and week(s) does this change affect?**
Module(s): ITP Structuring & Testing Data
Week(s): 3

**What is the work that needs to be done?**

The prep content gives a good example of a TDD workflow without ever explicitly saying that this is what we're doing. When trainees get to the backlog they are then given a ticket asking them to complete "TDD coursework". We should add some explanation of what we mean by TDD, either as an intro to the coursework or as part of the wider testing discussion.

**Why is this work important to do?**

- It's unfair to present trainees with an acronym and no context/explanation for it
- TDD is an important part of many development workflows, and there's a strong possibility trainees will need to use it in an interview setting.

**Additional context**

**Who might need to know about this change?**