Bidirectional streaming regression on TestServer

[JamesNK]

A customer attempted to use TestServer to test gRPC services. They weren't able to successfully test calls that use bidirectional streaming.

grpc/grpc-dotnet#624

This functionality worked during 3.0 dev. grpc-dotnet functional tests were run with TestServer. At some point after grpc-dotnet switched to using a local Kestrel instance for functional tests, bidirectional streaming in TestServer stopped working.

To Reproduce

1. git clone https://github.com/JamesNK/grpc-dotnet/tree/jamesnk/bidirectionalstreaming-repro
2. Run SayHelloBidirectionStreamingTest at https://github.com/JamesNK/grpc-dotnet/blob/c0a098ce8f44a751608ef1e9ef7ba1238d369aec/examples/Tester/Tests/FunctionalTests/GreeterServiceTests.cs#L101-L126
3. Test will hang on ResponseStream.MoveNext() (line 118)
4. Uncomment RequestStream.CompleteAsync() (line 116) and run test again
5. Test now passes

Expected behavior

TestServer should be able to return response content while request is still in progress.

Additional context

.NET Core SDK (reflecting any global.json):
 Version:   3.0.100
 Commit:    04339c3a26

[JamesNK]

The gRPC method on the server doesn't start at all until the request is completed.

[JamesNK]

Hmmm, I'm not sure this is a regression anymore. I was certain this use to work, but test server has called ReadAsStreamAsync() forever, and that blocks until the request content is complete.

[JunTaoLuo]

Looks like it's a 3.1 candidate.

[JamesNK]

Fixed #15591