doc: note that tests should include a description

Checklist

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

Affected core subsystem(s)

doc, test

Description of change

When debugging tests it is a huge help to have some basic information about what the purpose of the test actually is, especially as the person who originally wrote the test may no longer be active. This updates the Writing Tests guide to require that.

It's unlikely that anyone is going to have the time or the inclination to go through every test adding documentation, but specifying that we'd like some would be a good start.

Nits/bikeshedding/flat-out disagreement welcome!

cc/ @nodejs/testing