Add tests for channels::spsc module

[igorostrowskiq]

crate async_runtime
├── mod channels: pub
│   └── mod spsc: pub
│       ├── struct Receiver: pub
│       │   └── async fn recv: pub
│       ├── struct Sender: pub
│       │   └── fn send: pub
│       ├── fn create_channel: pub
│       └── fn create_channel_default: pub

The behavior should be similar to channels in well established crates like futures and Tokio. If our documentation is not clear, topics for improvement should be reported in https://github.com/qorix-group/inc_orchestrator_internal/issues/148