doc: improve testing guide

Add guide on choice of assertions, use of ES.Next features,
and the WPT upstream.

PR-URL: https://github.com/nodejs/node/pull/11150
Ref: https://github.com/nodejs/node/pull/11142


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>