code: def test_append(self):
      dd = DataSet()
      for _ in range(3):
          dd.append(Instance(x=[1, 2, 3, 4], y=[5, 6]))
      assert len(dd) == 3
      assert dd.field_arrays["x"].content == [[1, 2, 3, 4]] * 3
      assert dd.field_arrays["y"].content == [[5, 6]] * 3
  
imports: import os
  import pytest
  import numpy
  from fastNLP.core.dataset import DataSet, FieldArray, Instance, ApplyResultException
  from fastNLP import logger
label_desc: Use image node_id 1 for calling the TestDataSetMethods obj's underlying member method code with example usage: obj.test_append() without return types