message: ssl: avoid reaching into BoringSSL
  Rather than reaching into not_resumable, there's the
  SSL_SESSION_is_resumable API. (This is so a future revision of BoringSSL
  can hide the SSL_SESSION struct and make it C++.)
diff: @@ -217,7 +217,7 @@ const std::string testUtilV2(const envoy::api::v2::Listener& server_proto,
  EXPECT_EQ(expected_protocol_version, SSL_get_version(client_ssl_socket));
  }
  SSL_SESSION* client_ssl_session = SSL_get_session(client_ssl_socket);
  - EXPECT_FALSE(client_ssl_session->not_resumable);
  + EXPECT_TRUE(SSL_SESSION_is_resumable(client_ssl_session));
  uint8_t* session_data;
  size_t session_len;
  int rc = SSL_SESSION_to_bytes(client_ssl_session, &session_data, &session_len);
  @@ -998,7 +998,7 @@ void testTicketSessionResumption(const std::string& server_ctx_json1,
  .WillOnce(Invoke([&](Network::ConnectionEvent) -> void {
  Ssl::SslSocket* ssl_socket = dynamic_cast<Ssl::SslSocket*>(client_connection->ssl());
  ssl_session = SSL_get1_session(ssl_socket->rawSslForTest());
  - EXPECT_FALSE(ssl_session->not_resumable);
  + EXPECT_TRUE(SSL_SESSION_is_resumable(ssl_session));
  client_connection->close(Network::ConnectionCloseType::NoFlush);
  server_connection->close(Network::ConnectionCloseType::NoFlush);
  dispatcher.exit();
  @@ -1335,7 +1335,7 @@ TEST_P(SslSocketTest, ClientAuthCrossListenerSessionResumption) {
  .WillOnce(Invoke([&](Network::ConnectionEvent) -> void {
  Ssl::SslSocket* ssl_socket = dynamic_cast<Ssl::SslSocket*>(client_connection->ssl());
  ssl_session = SSL_get1_session(ssl_socket->rawSslForTest());
  - EXPECT_FALSE(ssl_session->not_resumable);
  + EXPECT_TRUE(SSL_SESSION_is_resumable(ssl_session));
  server_connection->close(Network::ConnectionCloseType::NoFlush);
  client_connection->close(Network::ConnectionCloseType::NoFlush);
  dispatcher.exit();
  