Add ActorOf overload that uses ActorDsl syntax

TestKit is in itself an `ActorRefFactory`, meaning that you inside a test can create an actor without having to go thru the `Sys` property:
``` C#
[Fact]
public void MyTest
{
    var a = ActorOf<MyActor>();
    ...
}
```

This PR adds an overload that allows usage of `IActorDsl`:
``` C#
[Fact]
public void MyTest
{
    var a = ActorOf(c =>
    {
        c.Receive<string>((s, context) => context.Sender.Tell("Hello " + s));
    });
    .
}
```