I think there are more cases for union.

The two ranges can be disjoint, they can have upper of one == lower of the other, partially overlap, partially overlap with an end-point in common, be equal, be a strict subset and unless you canonicalise the order somehow you also have all of these the other way round (so double the number of cases, plus some I missed probably).

I can think of at least ten cases anyway.

(Edit: after re-reading the tests, I think most cases are covered, so sorry for originally saying "a lot more cases", that was my error in reading the tests)