message: test: fix Response204WithBody flake
  Fixes
diff: @@ -1428,10 +1428,11 @@ TEST_P(IntegrationTest, Response204WithBody) {
  auto response = codec_client_->makeHeaderOnlyRequest(default_request_headers_);
  waitForNextUpstreamRequest();
  - // Create a response with a body
  + // Create a response with a body. This will cause an upstream messaging error but downstream
  + // should still see a response.
  upstream_request_->encodeHeaders(Http::TestResponseHeaderMapImpl{{":status", "204"}}, false);
  upstream_request_->encodeData(512, true);
  - ASSERT_TRUE(fake_upstream_connection_->close());
  + ASSERT_TRUE(fake_upstream_connection_->waitForDisconnect(true));
  response->waitForEndStream();
  