Restructure tests

[martinju]

Currently, the tests are complicated, hard to maintain and without a clear structure. I suggest to restructure the entire test base as follows:

• Basic small functions should be tested for giving correct input and output as currently is the case
• All other functions should be tested through the exported functions (mainly shapr and explain)

Other notes:

• For the ctree approach, we should test both the party package and the partykit fallback.
• Switch to using an example dataset that we either create/simulate and store within our package, we find a new data set that we can store in our package or we might use a dataset from the datasets package that we may not need to add to Suggests and use requireNameSpace for (see Switch test/example dataset #248 )
• If possible in a simple way, make sure that the tests actually ends up at the intended stop-/warning-message