doc: expand common module material in test guide

Provide a little more information about the `common` module. This
doesn't really provide much that wasn't already there, but hopefully
makes some things a bit more explicit for first-time readers.

PR-URL: https://github.com/nodejs/node/pull/10251


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>