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>

conn.go

@@ -1378,7 +1378,7 @@ func (c *Conn) ConnectionState() ConnectionState {
 		state.VerifiedChains = c.verifiedChains
 		state.SignedCertificateTimestamps = c.scts
 		state.OCSPResponse = c.ocspResponse
-		if !c.didResume {
+		if !c.didResume && c.vers != VersionTLS13 {
 			if c.clientFinishedIsFirst {
 				state.TLSUnique = c.clientFinished[:]
 			} else {