text: Title: tb contains method
  Body: > Also, it would be nice if the following check is supported:
  > 'happy_fraction' in tb
  
  Implement a `__contains__` method which returns whether the variable/function is present in the notebook or not (in memory).
  
  _Originally posted by @rohitsanj in https://github.com/nteract/testbook/issues/96#issuecomment-819540146_