doc: add test naming information to guide

The guide for writing tests is missing information on how tests are
named. This adds that information.

There is also some copy-editing done on the first paragraph of the
guide.

PR-URL: https://github.com/nodejs/node/pull/10584


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>