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

modules/dnn/test/test_onnx_conformance.cpp

@@ -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);
+    ASSERT_GE(outputs.size(), 1);
-    const std::vector<Mat>& outputs = outputs_[0];
 
     if (checkLayersFallbacks && checkFallbacks(net))
     {