docstr: Test closest_parent returns expected value and returns None for highest node in hierarchy.
code: def test_closest_parent(self):
      """Test closest_parent returns expected value and returns None for highest node in hierarchy."""
      self.assertEqual(
          self.vectors.closest_parent("dog.n.01"), "canine.n.02"
      )
      self.assertEqual(self.vectors.closest_parent("mammal.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 14 for calling the TestPoincareKeyedVectors obj's underlying member method code with example usage: obj.test_closest_parent() without return types