Move the version to an extension in the experimental TLS 1.3 encoding.

Change-Id: I0726e11006235db9309a8370a11e00ede0216279
Reviewed-on: https://boringssl-review.googlesource.com/17704
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/ssl/test/runner/common.go b/ssl/test/runner/common.go
index 2be474c..b40f222 100644
--- a/ssl/test/runner/common.go
+++ b/ssl/test/runner/common.go
@@ -1262,6 +1262,10 @@
 	// specified value in ServerHello version field.
 	SendServerHelloVersion uint16
 
+	// SendServerSupportedExtensionVersion, if non-zero, causes the server to send
+	// the specified value in supported_versions extension in the ServerHello.
+	SendServerSupportedExtensionVersion uint16
+
 	// SkipHelloRetryRequest, if true, causes the TLS 1.3 server to not send
 	// HelloRetryRequest.
 	SkipHelloRetryRequest bool