code: def test_add_field(self):
      fields = {"x": [1, 2, 3], "y": [4, 5, 6]}
      ins = Instance(**fields)
      ins.add_field("z", [1, 1, 1])
      fields.update({"z": [1, 1, 1]})
      assert ins.fields == fields
  
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 2 for calling the TestCase obj's underlying member method code with example usage: obj.test_add_field() without return types