Reorganize "Testing" pages
Created by: totten
Based on the evaluation/planning at the Devon sprint, we felt it was important to provide a better framing/summary of the kinds of tests that are available (and when to use them).
This PR reorganizes the content of the "Testing" chapter, migrates in content from civicrm-core
READMEs, and opens with a table to visualize the choice of testing techniques.
It's planned to put Codeception and Protractor in here as well, but the for the moment I've disabled those links.
(Comment updated Oct 30.)