Add SSL_get_tls_unique.

SSL_get_tls_unique returns the tls-unique channel-binding value as
defined in https://tools.ietf.org/html/rfc5929#section-3.1.

Change-Id: Id9644328a7db8a91cf3ff0deee9dd6ce0d3e00ba
Reviewed-on: https://boringssl-review.googlesource.com/4984
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/test/runner/common.go b/ssl/test/runner/common.go
index feef551..edebba1 100644
--- a/ssl/test/runner/common.go
+++ b/ssl/test/runner/common.go
@@ -188,6 +188,7 @@
 	VerifiedChains             [][]*x509.Certificate // verified chains built from PeerCertificates
 	ChannelID                  *ecdsa.PublicKey      // the channel ID for this connection
 	SRTPProtectionProfile      uint16                // the negotiated DTLS-SRTP protection profile
+	TLSUnique                  []byte
 }
 
 // ClientAuthType declares the policy the server will follow for