Fix queue worker incorrectly stopped when there are still more items in the queue...

Fix queue worker incorrectly stopped when there are still more items in the queue (#29532) (#29546)

Backport #29532

Without `case <-t.C`, the workers would stop incorrectly, the test won't
pass. For the worse case, there might be only one running worker
processing the queue items for long time because other workers are
stopped. The root cause is related to the logic of doDispatchBatchToWorker.
It isn't a serious problem at the moment, so keep it as-is.