Add testing for feat functions

[swanhl]

• Test on FEAT output correctness using a mocked up small dataset
• No test cases for perturbation & shapely feature tests, requires model input and results are not deterministic (involves randomness)

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[timlrx]

Can we change the example, to have a p-value < 1. Otherwise, these two test are not that meaningful. With two different thresholds, one should pass and the other fail.

[timlrx]

Both still have a p-value of 1

[timlrx]

For Perturbation and SHAPFeatureImportance, I recommend creating a simple model as input using one of scikitlearn test datasets https://scikit-learn.org/stable/datasets/toy_dataset.html and assert on the results.