[ci skip] Cleanup testing doc (#345)
[ci skip] Cleanup testing doc (#345)

* Mention about skipped tests in the testing doc
* Remove the content about fetching and exporting modules since it
does not belong to this doc.
* Mention about linting section being optional
* Mention about visual studio section being specific to that editor
and can be skipped otherwise.
exercism/xjavascript,exercism/xjavascript