text: Title: Convert some `unittest` tests to `pytest` style
  Body: Some of our tests follow the `unittest` structure of creating tests. While these are fine, we want to migrate our tests to be in the more functional style of `pytest`.
  
  For example, [`test_classification`](https://github.com/automl/auto-sklearn/blob/master/test/test_pipeline/test_classification.py) which tests the classification pipeline adopts the `unittest` style of class structured tests in which you must understand the whole setup to often understand one test. 
  
  In contrast, a more functional `pytest` looks like [`test_estimators.py::test_fit_performs_dataset_compression`](https://github.com/automl/auto-sklearn/blob/use_new_splitter/test/test_automl/test_automl.py#L989). Here we can define the test entirely through parameters, specify multiple parameter combinations, document which ones we expect to fail and give a reason for it. Because the test is completely defined through parameters, we can document each parameter and make the test almost completely-transparent by just looking at that one test. 
  
  We would appreciate any contributors on this side who want to get more familiar or are already familiar with testing. Some familiarity or willingness to learn `pytest`'s  `@parametrize` and `@fixture` features is a must. Please see the [contribution guide](https://github.com/automl/auto-sklearn/blob/master/CONTRIBUTING.md) if you'd like to get started!