HttpClient: Timing issue in sync ConnectHelper.Connect

[geoffkizer]

This code has a timing issue:

                using (cancellationToken.UnsafeRegister(static s => ((Socket)s!).Dispose(), socket))
                {
                    socket.Connect(new DnsEndPoint(host, port));
                }

If the socket.Connect call succeeds, but then the CancellationToken fires before the registration is disposed, then the Socket will be disposed but we will still continue as if the Connect succeeded.

I'm reasonably sure that's what caused this failure: https://helix.dot.net/api/2019-06-17/jobs/e63fcc11-e9ad-476c-9798-47f2f6d3b152/workitems/System.Net.Http.Functional.Tests/console

[ghost]

Tagging subscribers to this area: @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

[ManickaP]

This has been already fixed in #39514
But returned back, probably via bad resolution of conflicts in merge, in #39524.

I can fix that.