test: Update information on 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.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
test