Function spawn_with 

pub fn spawn_with<T, U, F, S>(f: F) -> (Spawn<S>, Handle<T, U>)
where
    F: Fn(Mock<T, U>) -> S,

Create a mock Service, pass it through f, and spawn the result on a mock task.

This is like spawn(), but the closure f may wrap the Mock in additional middleware before it is spawned.