Function test_stream 

pub async fn test_stream<L, R, E1, E2>(left: L, right: R)
where
    L: Transport<(), u32, E1> + Unpin,
    R: Transport<u32, (), E2> + Unpin,
    E1: Debug,
    E2: Debug,

Test transport by collecting received messages while treating transport as a stream.

It verifies if transport closing (or dropping) on one side is communicated to the other side - without it, stream would never complete.