ENH: `multicol` naive implementation. part2
follows #43369
@ivanovmg you fancy reviewing this one too? @ivanovmg changes made.

I disagree with your philosophy on logic in tests. Personally I prefer parametrizations to focus as much as possible on as few parameters, and be an indicator to the purpose of the test.
The purpose of the test is not really to check `siunitx` affects the column format and level1 header, it just happens to do this so the test has to be structured correctly. This test is small enough so it doesn't really matter either way but that is at least my reasoning.