crypto/tls: update testing certificates.

crypto/tls: update testing certificates.

This change alters the certificate used in many tests so that it's no
longer self-signed. This allows some tests to exercise the standard
certificate verification paths in the future.

Change-Id: I9c3fcd6847eed8269ff3b86d9b6966406bf0642d
Reviewed-on: https://go-review.googlesource.com/13244
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>