test: accept expected AIX result test-stdio-closed

AIX handles closed stdio differently (but still compliant with spec as
far as I can tell) than other POSIX variants we test. Test results are
different than Linux and others because AIX takes measures to not re-use
the file descriptors for stdio if one of the stdio streams is closed.

Fixes: https://github.com/nodejs/node/issues/8375
PR-URL: https://github.com/nodejs/node/pull/8755


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>