text: Title: Improve some code coverage
  Body: This is an ongoing issue we need to work on or would gladly accept contributions for anyone looking to get some experience with testing open-source. 
  
  * We have some guaranteed randomness with the configuration we tests which causes the code coverage to fluctuate. This causes issue in that we have a simple change such as a docstring change #1349 causes our [code coverage test to fail](https://github.com/automl/auto-sklearn/pull/1349/checks?check_run_id=4582381900), due to a `-0.03%` change in coverage.
      * We would still like some non-determinism in the configurations we try while testing, therefore we must *also* have deterministic tests that ensure all parts of our pipeline with all configurations are tested properly.
      
  * In general, we are hovering around `88%` coverage, these fluctuations are relatively minor and in no way account for the other `12%`. I would roughly estimate that we could quite easily reach `5%` with ensuring more components of the system are tested. I estimate the remaining `5%` is testing the various branches of `if/else`, error validation and maybe `~1-2%` untestable code we do not need to be concerned with (abstract classes etc...)
  
  Please check out our [contribution guide](https://github.com/automl/auto-sklearn/blob/master/CONTRIBUTING.md), `pytest-coverage` and our [code coverage statistics](https://app.codecov.io/gh/automl/auto-sklearn) reported from our automatic unittests if you'd like to get started!