test: avoid test-cluster-master-* flakiness

Removed reliance on worker exit before arbitrary timeout. Instead of failing
the test after 200 or 1000 ms wait indefinitely for child process exit. If
the test hangs the test harness global timeout will kick in and fail the test.

Note that if the orphaned children are not reaped correctly (in the absence
of init, e.g. Docker) the test will hang and the harness will fail it.

PR-URL: https://github.com/nodejs/node/pull/6531


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>