crypto/tls: set ServerName and unset TLSUnique in ConnectionState in TLS 1.3

Fix a couple overlooked ConnectionState fields noticed by net/http
tests, and add a test in crypto/tls. Spun off CL 147638 to keep that one
cleanly about enabling TLS 1.3.

Change-Id: I9a6c2e68d64518a44be2a5d7b0b7b8d78c98c95d
Reviewed-on: https://go-review.googlesource.com/c/148900
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

handshake_server_tls13.go

@@ -213,6 +213,7 @@ GroupSelection:
 		return errors.New("tls: invalid client key share")
 	}
 
+	c.serverName = hs.clientHello.serverName
 	return nil
 }