Fix queue test (#30646)

Fix #30643

The old test code is not stable due to the data-race described in the
TODO added at that time.

Make it stable, and remove a debug-only field from old test code.

modules/queue/workerqueue.go

@@ -40,8 +40,6 @@ type WorkerPoolQueue[T any] struct {
 	workerMaxNum    int
 	workerActiveNum int
 	workerNumMu     sync.Mutex
-
-	workerStartedCounter int32
 }
 
 type flushType chan struct{}