doc: update information on test/known_issues

* Since https://github.com/nodejs/node/pull/6559 known_issues
  does run on CI.
* Add some notes to explain the expectations around tests in
  known_issues.

PR-URL: https://github.com/nodejs/node/pull/12262


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>