test: fix test-cluster-worker-kill

[santigimeno]

Pull Request check-list

Please make sure to review and check all of these items:

• Does make -j8 test (UNIX) or vcbuild test nosign (Windows) pass with
  this change (including linting)?
• Is the commit message formatted according to CONTRIBUTING.md?

Affected core subsystem(s)

test

Description of change

Don't check that the disconnect event is emitted before the exit
event as the order is not guaranteed.
See failing test @ https://ci.nodejs.org/job/node-test-commit-linux/nodes=centos5-32/2648/tapTestReport/test.tap-133/

not ok 133 test-cluster-worker-kill.js
# 
# assert.js:89
# throw new assert.AssertionError({
# ^
# AssertionError: worker: 'exit' event before 'disconnect' event
# at Worker.<anonymous> (/home/iojs/build/workspace/node-test-commit-linux/nodes/centos5-32/test/parallel/test-cluster-worker-kill.js:80:12)
# at Worker.g (events.js:273:16)
# at emitTwo (events.js:100:13)
# at Worker.emit (events.js:185:7)
# at ChildProcess.<anonymous> (cluster.js:361:14)
# at ChildProcess.g (events.js:273:16)
# at emitTwo (events.js:100:13)
# at ChildProcess.emit (events.js:185:7)
# at Process.ChildProcess._handle.onexit (internal/child_process.js:204:12)

[cjihrig]

LGTM

[jbergstroem]

CI: https://ci.nodejs.org/job/node-test-commit/2604/

[jasnell]

LGTM

[jasnell]

Landed in 8d39481