docstr: Test closest_child returns expected value and returns None for lowest node in hierarchy.
code: def test_closest_child(self):
      """Test closest_child returns expected value and returns None for lowest node in hierarchy."""
      self.assertEqual(
          self.vectors.closest_child("dog.n.01"), "terrier.n.01"
      )
      self.assertEqual(
          self.vectors.closest_child("harbor_porpoise.n.01"), None
      )
  
imports: import logging
  import os
  import tempfile
  import unittest
  import numpy
  from gensim.models.poincare import PoincareRelations, PoincareModel, PoincareKeyedVectors
  from gensim.test.utils import datapath
label_desc: Use image node_id 13 for calling the TestPoincareKeyedVectors obj's underlying member method code with example usage: obj.test_closest_child() without return types