Document how GitLab maintains a clean testing environment for specs

The following discussion from !16516 (merged) should be addressed:

• @rspeicher commented on a discussion: (+2 comments)

  This is something we can flag up in the next backend call. Perhaps it deserves a mention in the testing guidelines too? WDYT?

  @nick.thomas I think both would be a good idea. Documentation can be a separate MR unless you want to sneak it in here after review. 👍

Update the testing guidelines to include a section on how we maintain a clean testing environment, and pitfalls to watch out for.