message: test: fix TCP tunneling flake
  We can't full close the connection before we are done.
  Fixes
diff: @@ -476,11 +476,11 @@ TEST_P(TcpTunnelingIntegrationTest, TcpProxyUpstreamFlush) {
  ASSERT_TRUE(upstream_request_->waitForHeadersComplete());
  upstream_request_->encodeHeaders(default_response_headers_, false);
  upstream_request_->readDisable(true);
  - upstream_request_->encodeData("", true);
  + upstream_request_->encodeData("hello", false);
  // This ensures that fake_upstream_connection->readDisable has been run on its thread
  // before tcp_client starts writing.
  - tcp_client->waitForHalfClose();
  + ASSERT_TRUE(tcp_client->waitForData(5));
  ASSERT_TRUE(tcp_client->write(data, true));
  @@ -490,6 +490,7 @@ TEST_P(TcpTunnelingIntegrationTest, TcpProxyUpstreamFlush) {
  upstream_request_->readDisable(false);
  ASSERT_TRUE(upstream_request_->waitForData(*dispatcher_, size));
  ASSERT_TRUE(upstream_request_->waitForEndStream(*dispatcher_));
  + upstream_request_->encodeData("world", true);
  tcp_client->waitForHalfClose();
  }
  