Add Pipelines-related http features in TestServer

[analogrelay]

Epic #8829
Split from #7815

We should support the Pipe Goodness on TestServer.

We'd remove the stream-based implementation and use pipes. Stream-based implementation would then be a wrapper around the pipes.

[JamesNK]

Something to test/implement with this issue: ensure that Response.StartAsync correctly sets Response.HasStarted to true in TestServer. Right now I don't think calling flush on the response stream, e.g. Response.Body.FlushAsync sets it correctly.

[analogrelay]

Bumping out of preview4. I don't think this is a critical thing for preview 4, but feel free to debate :).

[JamesNK]

I would like it. Does that make it critical? 🙃

Seriously though, I was looking into grpc/grpc-dotnet#144 yesterday and TestServer never sets Response.HasStarted when the response content is flushed, and that makes our functional tests a little less trustworthy. For example, it is not an error to send headers after writing content. I'm considering moving some of the functional tests to use a spun up Kestrel instance because I don't trust how accurate TestServer is.

But I understand there is higher priority work.

[analogrelay]

We just don't have the capacity for this in 3.0. I'll put this in preview7 for now, and if someone feels so inspired, we can review a PR :)

[analogrelay]

@JamesNK does your PR satisfy this need? Are we ready to close this?

[JamesNK]

My PR satisfies my need to have StartAsync work correctly.

TestServer still needs to be refactored to use pipes, but from my POV you can bump it to post 3.0 if you don't have time.

[analogrelay]

Ok, we'll keep this around. There may be capacity later when the unknown unknowns become known knowns.

[analogrelay]

We don't expect to have the capacity in 3.0. Lots on the queue. This is a good candidate for 3.1 though.

[alefranz]

I had a look at this and I believe that most of work has been done as part of #11598.

[analogrelay]

Looks like you're correct and #11598 has a better listing of what is done and not yet done. I'm going to close this as a duplicate of it.