Add SSL_get_tls_unique.

SSL_get_tls_unique returns the tls-unique channel-binding value as
defined in https://tools.ietf.org/html/rfc5929#section-3.1.

Change-Id: Id9644328a7db8a91cf3ff0deee9dd6ce0d3e00ba
Reviewed-on: https://boringssl-review.googlesource.com/4984
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/test/test_config.cc b/ssl/test/test_config.cc
index df8553c..363b6f3 100644
--- a/ssl/test/test_config.cc
+++ b/ssl/test/test_config.cc
@@ -81,6 +81,7 @@
   { "-use-export-context", &TestConfig::use_export_context },
   { "-reject-peer-renegotiations", &TestConfig::reject_peer_renegotiations },
   { "-no-legacy-server-connect", &TestConfig::no_legacy_server_connect },
+  { "-tls-unique", &TestConfig::tls_unique },
 };
 
 const Flag<std::string> kStringFlags[] = {