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
If this charter overlaps with the Build WG charter, then the Build WG will also have to ratify it. @nodejs/build