Sink mock in tokio_test

[barafael]

Problem Description

I'm happily using the StreamMock from tokio_test. However, a corresponding Sink mock is missing.

Suggestion

I'd like to have an easy-to-setup Sink type as a mock.

Alternatives

I am not aware of simple alternatives.

Context

I would consider attempting to implement the sink mock.

[Darksonn]

Feel free to submit PRs for this.

[barafael]

#7190