Allow server supported_curves in TLS 1.3 in Go.

Change-Id: I1132103bd6c8b01c567b970694ed6b5e9248befb
Reviewed-on: https://boringssl-review.googlesource.com/8816
Reviewed-by: Nick Harper <nharper@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/test/runner/handshake_messages.go

@@ -1115,6 +1115,11 @@ func (m *serverExtensions) unmarshal(data []byte, version uint16) bool {
 				return false
 			}
 			// Ignore this extension from the server.
+		case extensionSupportedCurves:
+			// The server can only send supported_curves in TLS 1.3.
+			if version < VersionTLS13 || !enableTLS13Handshake {
+				return false
+			}
 		default:
 			// Unknown extensions are illegal from the server.
 			return false