code: def test_matmul(self):
      x = cupy.array([3, 7])
      y = MockArray2()
      assert x @ y == "rmatmul"
      assert y @ x == "matmul"
  
imports: import contextlib
  import unittest
  import numpy
  import pytest
  import cupy
  from cupy import testing
  import cupyx
label_desc: Use image node_id 2 for calling the TestArrayUfuncOptout obj's underlying member method code with example usage: obj.test_matmul() without return types