investigate flaky parallel/test-pipe-unref

[Trott]

• Version: 10.0.0-pre
• Platform: freebsd10-64
• Subsystem: test

https://ci.nodejs.org/job/node-test-commit-freebsd/13071/nodes=freebsd10-64/console:

not ok 1219 parallel/test-pipe-unref
  ---
  duration_ms: 1.978
  severity: fail
  stack: |-
    assert.js:42
      throw new errors.AssertionError({
      ^
    
    AssertionError [ERR_ASSERTION]: function should not have been called
        at Timeout.mustNotCall [as _onTimeout] (/usr/home/iojs/build/workspace/node-test-commit-freebsd/nodes/freebsd10-64/test/common/index.js:573:12)
        at ontimeout (timers.js:478:11)
        at Timer.unrefdHandle (timers.js:598:5)