[Unit Test Extension]: Extend "default setting unit tests" to methods

[MalteKurz]

We do have unit tests for model defaults, see https://github.com/DoubleML/doubleml-for-py/blob/master/doubleml/tests/test_doubleml_model_defaults.py. The intention behind such "default setting unit tests" is twofold:

1. It should assert that the defaults are valid / meaningful, i.e., the code runs through successfully with default values for the input parameters.
2. The unit tests serve as a reminder to update the documentation of defaults in case a default value is being changed.

Currently, we only have such "default setting unit tests" for the initialization of the model classes but it would also make sense to extend it to the most important methods.