Add tests using testthat about shapr HOT 2 OPEN

I am getting a bit worried that every time we make a change to shap or explain functions, we have to re-create the test objects (explanation_explain_obj.rds and similar) which compares the results of the previous run to pass the tests. That means we don't have a good test for this. This is especially dangerous if one does many changes in the same PR (although we seldom do that).

My suggestion is to add a seperate test that checks only the shapley values from the different test models. We could run a lapply call to the ex_list, extract only the data.table with the shapley values, and then do a testthat::expect_known_value or testthat::expect_known_object on that. What do you think @nikolase90 ? Or is there a better option?

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@martinju I think that sounds like a good way to do it.

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