test: fix flaky test-child-process-emfile

Require the test setup to obtain an EMFILE error and not ENFILE as
ENFILE means there is a race condition with other processes that may
close files before `spawn()` is called by the test.

Fixes: https://github.com/nodejs/node/issues/2666
PR-URL: https://github.com/nodejs/node/pull/3430


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>