crypto/tls: fix printing of OpenSSL output when updating a test fails.

crypto/tls: fix printing of OpenSSL output when updating a test fails.

When updating the test data against OpenSSL, the handshake can fail and
the stdout/stderr output of OpenSSL is very useful in finding out why.

However, printing that output has been broken for some time because its
no longer sent to a byte.Buffer. This change fixes that.

Change-Id: I6f846c7dc80f1ccee9fa1be36f0b579b3754e05f
Reviewed-on: https://go-review.googlesource.com/30823
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>