test,doc: document where common modules go

Keep the require('../common') separate from other common modules, as it's the only line that must be there.

I noticed that in the code-and-learn PRs, people are putting the require('../common/fixtures') require in with the normal ones. I think this is the right place to put it, but I'm open to suggestions.

Also the fixtures module isn't actually used in this example test, I could probably find another test that uses it, but will bloat the diff for questionable gain (making the lint pass on the sample test).

cc/ @nodejs/testing

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

test