Clippy Book Chapter Updates Reborn: Writing tests
This PR adds a new chapter to the book: "Writing tests". The changes have been mainly done from reviews from #9426 and some minor re-writes.
## Notes
- We still need to check that the `git status`es are correct, as `cargo dev new_lint` changed a lot since 2022.
- Requires #10598: Link to "Emitting Lints" where I flagged with `FIXME:`.
- To talk about the whole project, please use the tracking issue for the project #10597 (It also contains a timeline, discussions and more information)
changelog: Add a new "Writing tests" chapter to the book
r? `@flip1995`
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