Consider Receptable in TestKit

It is UNSAFE to do stuff like:

```swift
// pseudo code

func mytest() {
var myMutableThing = ..

system.spawn(.receive { _ in
myMutableThing.mutate() // UNSAFE DONT DO THIS
}).tell("hi")
}

testkit.eventually {
}
```

For simple types it may actually happen to accidentally sometimes work, but in general this is concurrent access to mutable data and is therefore unsafe in general.

We could offer:

```swift
let receptacle = testKit.receptacle(of: Thing.self)

system.spawn(.receive { _ in
receptacle.set("foo")
}).tell("hi")

await receptacle.expect("foo", within: .seconds(2))
```

it would mean people don't have to get out NIO just to test some actor thing

Start by locating TestKit and its existing eventually API, then review how actor callbacks interact with shared mutable test state. Done means TestKit offers a receptacle-based way to record and await values without requiring NIO, with coverage for the proposed usage.

Written by the indexing model from the issue text.