Akka FSM:如何编写测试用例来验证状态更改？

import akka.actor.ActorSystem
import akka.testkit.{ImplicitSender, TestFSMRef, TestKit}
import org.scalatest.{BeforeAndAfterAll, Matchers, WordSpecLike}

// 定义状态
sealed trait OrderState
case object PendingPayment extends OrderState
case object Paid extends OrderState
case object Shipped extends OrderState
case object Completed extends OrderState

// 定义消息
sealed trait OrderEvent
case object PaymentReceived extends OrderEvent
case object OrderShipped extends OrderEvent
case object OrderCompleted extends OrderEvent

// 定义订单状态机
class OrderFSM extends akka.actor.FSM[OrderState, Unit] {
  startWith(PendingPayment, Unit)

  when(PendingPayment) {
    case Event(PaymentReceived, _) =>
      goto(Paid)
  }

  when(Paid) {
    case Event(OrderShipped, _) =>
      goto(Shipped)
  }

  when(Shipped) {
    case Event(OrderCompleted, _) =>
      goto(Completed)
  }

  initialize()
}

// 测试用例
class OrderFSMSpec extends TestKit(ActorSystem("OrderFSMSpec"))
  with ImplicitSender
  with WordSpecLike
  with Matchers
  with BeforeAndAfterAll {

  override def afterAll: Unit = {
    TestKit.shutdownActorSystem(system)
  }

  "OrderFSM" should {
    "transition from PendingPayment to Paid when PaymentReceived event is received" in {
      val fsm = TestFSMRef(new OrderFSM)
      fsm ! PaymentReceived
      fsm.stateName should be(Paid)
    }
  }
}