changefeedccl: fix deadlock in mocksyncproducer

TestChangefeedKafkaMessageTooLarge would flake sometimes with goroutines
blocked on sending into the mocksyncproducer feedCh and would result in
a "Slow on quiesce" error. This is likely due to the jobFeed shutting
down and therefore no longer reading from the feedCh.

Release note: None

pkg/ccl/changefeedccl/testfeed_test.go

@@ -1608,7 +1608,10 @@ func (s *fakeKafkaSink) Dial() error {
 						return err
 					}
 				}
-				s.feedCh <- m
+				select {
+				case s.feedCh <- m:
+				case <-s.tg.done:
+				}
 				return nil
 			},
 		}}, nil