Add POD documentation to TestLib.pm

This module was pretty much undocumented.  Fix that.

Inspired by a preliminary patch sent by Ramanarayana, heavily updated by
Andrew Dunstan, and reviewed by Michael Paquier.

Discussion: https://postgr.es/m/CAF6A77G_WJTwBV9SBxCnQfZB09hm1p1O3stZ6eE5QiYd=X84Jg@mail.gmail.com

Author: alvherre