docs: Update testing configuration documentation for clarity (#7683)
### Description
The Testing section is really helpful, but it would be clearer by
specifying exactly where each of these snippets should be placed.
Before:
<img width="400" alt="Screenshot 2024-03-09 at 8 08 59 AM"
src="https://github.com/vercel/turbo/assets/26635607/1e8e8b05-4702-402b-8ea8-df7d9e6f21cf">
After:
<img width="400" alt="Screenshot 2024-03-09 at 8 14 03 AM"
src="https://github.com/vercel/turbo/assets/26635607/e6ab7ce4-0cb7-4f4d-b14d-9b70598ce7c3">
Looking forward to your thoughts and feedback.
Thanks.