[Messenger][FrameworkBundle] Add MessengerAssertionsTrait to test in-memory transports

A functional test that wants to run the messages queued on an in-memory
transport has to build a Worker by hand, with an event dispatcher and a
StopWorkerOnMessageLimitListener. FrameworkBundle ships assertion traits
for Mailer, Notifier, HttpClient and Console but none for Messenger.

Messenger:
- StopWorkerOnIdleListener stops the worker as soon as its receivers
  return no message, so a worker can drain a queue and exit without
  sleeping.
- InMemoryTransport implements ListableReceiverInterface and
  MessageCountAwareInterface: all(), find() and getMessageCount() expose
  the queued envelopes, delayed ones included. The messenger:show and
  messenger:stats commands work with in-memory transports as a result.

FrameworkBundle:
- MessengerAssertionsTrait, composed into KernelTestCase, provides
  assertQueuedMessageCount(), getQueuedMessages(), getMessengerTransport()
  and consumeQueuedMessages(). The last one runs the queued messages
  through the real message bus with the application's event dispatcher,
  so the retry and failure transport listeners behave as in production,
  and rethrows the first handler failure so that it fails the test.
- Listing and consuming queued messages need symfony/messenger 8.2. With
  an older version the trait throws a LogicException that says so, like
  the other Messenger 8.2 features of the bundle.