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.
This test as is had failed 6 out of 26 times that are in the Jenkins history for freebsd101-32 at https://ci.nodejs.org/job/node-test-commit-freebsd/. With the fix, it has run successfully 12 out of 12 times. (UPDATE: 14 out of 14 now.)