FairGBM: add tests to ensure FairGBM improves fairness on a small test dataset

[AndreFCruz]

Originally posted by @AlbertoEAF in #116 (comment):

We have no tests ensuring what we want to achieve - fairness :p

Yes, we can be using FGBM in the C++ part but let's say we have some bug in the provider and we don't pass the adequate parameters to it. It'll behave as LGBM. It'll happen and we have no clue.

We need to have a test ensuring we achieve a higher fairness than LGBM and sacrifice recall for it.