docs: add reviewing chapter to testing.md
docs: add reviewing chapter to testing.md

Cover some best practices while reviewing a test suite, as well as how
to run tests locally without impacting your current environment.

Tested: Viewed in Chrome

Change-Id: I7356a90fcc9e6d2679e1f9e9a49515a1038a8914
Signed-off-by: Emily Shaffer <a6e326c88a51217312e91af047fa468d2d7dea8f@google.com>

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