We now have unit, functional, and tempest tests in tree. To avoid
confusion, we need to make sure it's clear what each of these test
types are intended to be used for.
This adds a description for each test type and cleans up some of
the documentation around testing in general.
Change-Id: I15bba187889e058adbf03deb4308a41d0a6248e3
61238e2e54