doc: add documentation for test common module

Alphabetized list of methods and properties for the common.js module.
Add table of contents to the top of the readme.

PR-URL: https://github.com/nodejs/node/pull/8840


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>