Add the certificate_required alert.

This is part of TLS 1.3 draft 16 but isn't much of a wire format change,
so go ahead and add it now. When rolling into Chromium, we'll want to
add an entry to the error mapping.

Change-Id: I8fd7f461dca83b725a31ae19ef96c890d603ce53
Reviewed-on: https://boringssl-review.googlesource.com/11563
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/ssl/test/runner/handshake_server.go b/ssl/test/runner/handshake_server.go
index 7c2fd17..abadf3a 100644
--- a/ssl/test/runner/handshake_server.go
+++ b/ssl/test/runner/handshake_server.go
@@ -830,7 +830,7 @@
 			// The client didn't actually send a certificate
 			switch config.ClientAuth {
 			case RequireAnyClientCert, RequireAndVerifyClientCert:
-				c.sendAlert(alertBadCertificate)
+				c.sendAlert(alertCertificateRequired)
 				return errors.New("tls: client didn't provide a certificate")
 			}
 		}