test: improve test-cluster-disconnect-suicide-race

Previously, test-cluster-disconnect-suicide-race had two issues:

* Magic numbers: How many times to spawn a worker was determined through
empirical experimentation. This means that as new platforms and new
CPU/RAM configurations are tested, the magic numbers require more
and more refinement. This brings us to...

* Non-determinism: The test seems to fail all the time when the bug
it tests for is present, but it's really a judgment based on sampling.
"Oh, with 8 workers per CPU, it fails about 80% of the time. Let's try
16..."

This revised version of the test takes a different approach. The fix
for the bug that the test was written for means that the disconnect
event will fire on a subsequent tick. So we check for that and the test
still fails when the fix is not in the code base and succeeds when it
is.

Advantages of this approach include:

* The test runs much faster.
* The test should be reliable on any new platform regardless of CPU and
RAM.

PR-URL: https://github.com/nodejs/node/pull/4739
Ref: https://github.com/nodejs/node/pull/4674


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