fix rare spurios make test failures in new_concurrency_throttle

Which was apparently caused by race between us killing test worker
processes and throttle detecting it. We now wait for childs death
prior to capturing throttle ets tables contents.

Change-Id: Ie964dd4b7aebf1483644d572bf96d459a665b56a
Reviewed-on: http://review.couchbase.org/39626
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>