Add CMAKE_FIND_ROOT_PATH test

Okay, it seems I have to create another Pull Request, but nevermind. This is a followup to #2459.

I am unsure if this test even works, because I didn't find a test called "cmake". Maybe I'm just doing something wrong, but the test code should be okay.

I don't expect this to be accepted yet, someone probably has to tell me how to make that test runnable first, maybe the cmake stuff is just missing in `test_core.py`...