Unstashed messages enqueued in TestActorRef mailbox while processing a message from self are stuck until another message is received

This is a bug created on the pekko side for the issue here: https://github.com/akka/akka/issues/30170

In the following workflow, unstashed messages are enqueued in the mailbox but are not processed as expected:

1. Send message (`messageOne`) from TestKit to TestActorRef
2. TestActorRef underlying actor stashes `messageOne` and schedules a subsequent message to self (`selfMessage`)
3. `selfMessage` is sent and processed and `messageOne` is unstashed during processing of `selfMessage`
4. `messageOne` is not processed
5. Send another message (`messageTwo`) from TestKit to TestActorRef
6. Both `messageOne` and `messageTwo` are processed

`akka.testkit.CallingThreadDispatcher#runQueue` appears to process the provided `MessageQueue` at the time of invocation rather than using the active `Mailbox` `MessageQueue`.