code: def test_get_field_names(self):
      ds = DataSet({"x": [[1, 2, 3, 4]] * 40, "y": [[5, 6]] * 40})
      field_names = ds.get_field_names()
      assert "x" in field_names
      assert "y" in field_names
  
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 14 for calling the TestDataSetMethods obj's underlying member method code with example usage: obj.test_get_field_names() without return types