doc: add Testing WG

Add the proposed Testing WG. WORKING_GROUPS.md indicates that opening
a pull request to that file is the way to request that a charter be
ratified by the TC. So, that's what this is.

The charter document is currently:
https://github.com/nodejs/testing/blob/master/Charter.md

PR-URL: #5461
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Alexis Campailla <orangemocha@nodejs.org>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>

WORKING_GROUPS.md

@@ -32,6 +32,7 @@ back in to the CTC.
 * [Intl](#intl)
 * [HTTP](#http)
 * [Documentation](#documentation)
+* [Testing](#testing)
 
 #### Process:
 
@@ -287,6 +288,18 @@ Its responsibilities are:
 * Creating and operating a process for documentation review that produces
   quality documentation and avoids impeding the progress of Core work.
 
+### [Testing](https://github.com/nodejs/testing)
+
+The Node.js Testing Working Group's purpose is to extend and improve testing of
+the Node.js source code.
+
+It's responsibilities are:
+
+* Coordinating an overall strategy for improving testing.
+* Documenting guidelines around tests.
+* Working with the Build Working Group to improve continuous integration.
+* Improving tooling for testing.
+
 ## Starting a WG
 
 A Working Group is established by first defining a charter  that can be