code: def test_output(self):
      self.expect(self.model, (self.x1, self.x2))
  
imports: import chainer
  import chainer.functions
  from chainer import testing
  import numpy
  import onnx
  import pytest
  from onnx_chainer import export
  from onnx_chainer.testing import input_generator
  from onnx_chainer_tests.helper import ONNXModelChecker
  from onnx_chainer_tests.helper import ONNXModelTest
label_desc: Use image node_id 2 for calling the TestConcat obj's underlying member method code with example usage: obj.test_output() without return types