Create StreamMock for testing Streams

[shade]

Inroduce a new mock type to tests streams and eventually sinks. Only includes next() and wait() for now.

Motivation

Basically making a generic Stream mock type #4106

Next PR I'd like to add a Sink implementation. Since Sink requires an error type, I think we can do something like this.

impl Sink<T> for StreamMock<T> {
// Consume implementation
}

impl <T: Unpin, E: Error> Sink<T> for StreamMock<Result<T,E>> {
   type Error = E;
}

Main motivation is so we can support splitting the stream mock into a AsyncRead/AsyncWrite.