text: Title: Extend unit tests with Hypothesis framework
  Body: <!-- Instructions For Filing a Bug: https://github.com/giotto-learn/giotto-learn/blob/master/CONTRIBUTING.rst -->
  
  #### Description
  <!-- Example: Joblib Error thrown when calling fit on VietorisRipsPersistence 
  -->
  Instead of using hard-coded examples in our unit tests, we could automatically generate a range of "strategies" using the Hypothesis test framework:
  
  * [Hypothesis](https://hypothesis.readthedocs.io/en/latest/index.html)
  * [hypothesis-gufunc](https://github.com/uber/hypothesis-gufunc)
  
  This would allow us to catch a wider range of edge cases.
  