Add test for renego client_version quirk.

In upstream's f4e1169341ad1217e670387db5b0c12d680f95f4, the client_version was
made constant across renegotiations, even if the server negotiated a lower
version. NSS has the same quirk, reportedly for SChannel:

https://code.google.com/p/chromium/codesearch#chromium/src/net/third_party/nss/ssl/ssl3con.c&sq=package:chromium&l=5103

Add a test to ensure we do not regress this.

Change-Id: I214e062463c203b86a9bab00f8503442e1bf74fe
Reviewed-on: https://boringssl-review.googlesource.com/2405
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/test/runner/conn.go b/ssl/test/runner/conn.go
index 94d7434..177e458 100644
--- a/ssl/test/runner/conn.go
+++ b/ssl/test/runner/conn.go
@@ -58,6 +58,8 @@
 
 	srtpProtectionProfile uint16
 
+	clientVersion uint16
+
 	// input/output
 	in, out  halfConn     // in.Mutex < out.Mutex
 	rawInput *block       // raw input, right off the wire