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: https://github.com/nodejs/node/pull/5461


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