Fix flaky Kestrel Http/2 Stream tests

[BrennanConroy]

We use inline pipe scheduling which means the StartStreamAsync call will call continuations inline which will add the stream to the connection before returning. However, inline continuations relies on a continuation actually being present. So all these tests relied on a race condition where we had an active pipeReader.ReadAsync call in order to call the continuation inline. If you add a delay or pause the thread right before the ReadAsync in Http2Connection.cs then you can force the race and cause _connection._streams[1]; to fail as there is no stream present yet.

The fix is to make sure the stream is present, and we attain that by making sure the app code is running which is after the stream has been created.