Add test for windup problem in APF

The current design for Fair Queueing for Server Requests has a
problem: if the min-max fair result stays different from an even
division for a long time and no queue involved in the imbalance goes
empty then the imbalance keeps accruing in queue virtual state times.

This commit adds a test that demonstrates the problem.

It also has some other tweaks to make other tests less flaky.

Factor the big scenario-testing func into pieces, with supporting
structs.

Author: MikeSpreitzer

staging/src/k8s.io/apiserver/pkg/util/flowcontrol/fairqueuing/queueset/queueset.go

@@ -441,7 +441,7 @@ func (qs *queueSet) chooseQueueIndexLocked(hashValue uint64, descr1, descr2 inte
 			bestQueueIdx, bestQueueLen = queueIdx, thisLen
 		}
 	})
-	klog.V(6).Infof("QS(%s): For request %#+v %#+v chose queue %d, had %d waiting & %d executing", qs.qCfg.Name, descr1, descr2, bestQueueIdx, bestQueueLen, qs.queues[bestQueueIdx].requestsExecuting)
+	klog.V(6).Infof("QS(%s) at r=%s v=%.9fs: For request %#+v %#+v chose queue %d, had %d waiting & %d executing", qs.qCfg.Name, qs.clock.Now().Format(nsTimeFmt), qs.virtualTime, descr1, descr2, bestQueueIdx, bestQueueLen, qs.queues[bestQueueIdx].requestsExecuting)
 	return bestQueueIdx
 }