Add Simple Tests

[wakame1367]

I made a simple test with reference to the test in this Link.

[tonyduan]

Hi! Thank you for the contribution. Wanted to give you a heads up I've pushed some minor changes to the master branch. The most significant API change is that the Score is now static, i.e. instead of
ngb = NGBoost(Base=default_tree_learner, Dist=Normal, Score=MLE(),
an instantiation would look like
ngb = NGBoost(Base=default_tree_learner, Dist=Normal, Score=MLE,.

Sorry for the inconvenience and thanks again!

[wakame1367]

Hi! Thank you for the contribution. Wanted to give you a heads up I've pushed some minor changes to the master branch. The most significant API change is that the Score is now static, i.e. instead of
ngb = NGBoost(Base=default_tree_learner, Dist=Normal, Score=MLE(),
an instantiation would look like
ngb = NGBoost(Base=default_tree_learner, Dist=Normal, Score=MLE,.

Sorry for the inconvenience and thanks again!

OK, I reflected it👌