[async/await] Add async await support to ActorTestProbe

For now let's keep all the APIs but allow them to async/await rather than block the current thread.

E.g. it should be possible to:

```
let probe = testKit.spawnTestProbe(...)

try await probe.expectMessage()
```

rather than the blocking version of this call

Start by locating ActorTestProbe and its blocking expectMessage API, then trace the other probe APIs that should keep their existing behavior. Check how the test kit currently waits for messages and define completion as supporting try await calls without removing the blocking APIs.

Written by the indexing model from the issue text.