Documentation: dev-tools: Add Testing Overview

The kernel now has a number of testing and debugging tools, and we've
seen a bit of confusion about what the differences between them are.

Add a basic documentation outlining the testing tools, when to use each,
and how they interact.

This is a pretty quick overview rather than the idealised "kernel
testing guide" that'd probably be optimal, but given the number of times
questions like "When do you use KUnit and when do you use Kselftest?"
are being asked, it seemed worth at least having something. Hopefully
this can form the basis for more detailed documentation later.

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Link: https://lore.kernel.org/r/20210415054036.581117-1-davidgow@google.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>