docs: add initial testing.md outline
docs: add initial testing.md outline

Add a new piece of documentation, intended to be a tutorial on running,
writing, and reviewing unit tests for the CI suite. Outline the sections
to be written in some detail (subject to change).

There are a handful of methods for running the CI, foremost by using
Docker or by building the OpenBMC SDK. This tutorial will cover the
Docker method, as it is most similar to what the Jenkins bot does, and
most consistent across platforms.

Tested: Viewed markdown file in Chrome.

Change-Id: Ic98c02bc8f4dc6eaf4e7d1432422cdd934862548

openbmc/phosphor-host-ipmid,openbmc/phosphor-host-ipmid,openbmc/phosphor-host-ipmid