text: Title: Docstring for AllenNlpTestCase is out-of-date
  Body: https://github.com/allenai/allennlp/blob/ae7cf85b8f755f086341a087aeca1ba7c1df3769/allennlp/common/testing/test_case.py#L14
  
  This docstring suggests that the TestCase class should subclass `unittest.TestCase` but it does not do so. Should this be updated or is this intentional? 