Attempt to prevent the deadlock in the QueueDiskChannel Test again (#18415)

* Attempt to prevent the deadlock in the QueueDiskChannel Test again

This time we're going to adjust the pause tests to only test the right
flag.

* Only switch off pushback once we know that we are not pushing anything else
* Ensure full redirection occurs
* More nicely handle a closed datachan
* And handle similar problems in queue_channel_test

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/queue/unique_queue_channel.go

@@ -178,7 +178,10 @@ func (q *ChannelUniqueQueue) FlushWithContext(ctx context.Context) error {
 		default:
 		}
 		select {
-		case data := <-q.dataChan:
+		case data, ok := <-q.dataChan:
+			if !ok {
+				return nil
+			}
 			if unhandled := q.handle(data); unhandled != nil {
 				log.Error("Unhandled Data whilst flushing queue %d", q.qid)
 			}