test: check fd 0,1,2 are used, not access mode

Don't do a write on stdout/stderr because that checks for their
writability. But fd=1 could legitimately be opened with read-only
access by the user. All this test needs to ensure is that
they are used at startup.

PR-URL: https://github.com/nodejs/node/pull/10339
Fixes: https://github.com/nodejs/node/issues/10234


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>