Can this custom Test:Unit assert be replaced/refactored? Am I missing test cases?
You have adequate test coverage for integers, and probably this will extend to floating points as well (although you really should test those as well, IMO). However you should also test for objects. Also, try a couple combinations of objects, numbers, and strings to be really sure that your assertion is correct.

If you only need to test for integer arrays, you should make it clear that the assert only works for them and its behavior with other arrays is undefined. In that case maybe you should change the name to assert_same_items_integer or something similar for clarity.