test: fix unreliable known_issues test

`test-stdout-buffer-flush-on-exit` was not failing reliably on POSIX
machines and not failing at all on Windows. Revised test fails reliably
on POSIX and is skipped (in CI) on Windows where the issue does not
exist.

Fixes: https://github.com/nodejs/node/issues/6527
PR-URL: https://github.com/nodejs/node/pull/6555


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joao Reis <reis@janeasystems.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>