message: dnn(test): fix outputs handling in ONNX conformance
  ONNX output is 1 tensor per defined output instead of N tensors from outputs of "output" layer
diff: @@ -1171,10 +1171,10 @@ TEST_P(Test_ONNX_conformance, Layer_Test)
  }
  std::vector<String> layerNames = net.getUnconnectedOutLayersNames();
  - std::vector< std::vector<Mat> > outputs_;
  + std::vector<Mat> outputs;
  try
  {
  - net.forward(outputs_, layerNames);
  + net.forward(outputs, layerNames);
  }
  catch (...)
  {
  @@ -1182,8 +1182,7 @@ TEST_P(Test_ONNX_conformance, Layer_Test)
  applyTestTag(CV_TEST_TAG_DNN_ERROR_FORWARD);
  throw;
  }
  - ASSERT_GE(outputs_.size(), 1);
  - const std::vector<Mat>& outputs = outputs_[0];
  + ASSERT_GE(outputs.size(), 1);
  if (checkLayersFallbacks && checkFallbacks(net))
  {
  