Adds an error channel to the test helper function testPortForward() to collect errors that happen inside a goroutine. Change-Id: I6db1d24b935fdfad637c971581ae80beaebd8a1f Reviewed-on: https://go-review.googlesource.com/c/crypto/+/207462 Run-TryBot: Han-Wen Nienhuys <hanwen@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>