From time to time, test-vm-sigint.js fails on CI with no indication of what went wrong.

https://ci.nodejs.org/job/node-test-commit-linux/4202/nodes=centos5-32/console:

not ok 1066 parallel/test-vm-sigint
  ---
  duration_ms: 0.348

It's not timing out, but there's no assertion error either. It's just exiting with a bad code, I think?

/cc @addaleax