I appreciate we could test this in a million ways and I feel a light touch approach is pragmatic on this occasion. However, could you add a ```test_single_cube``` such as:
```python
def test_single_cube(self):
    res = CubeList([self.cube1]).merge_cube()
    self.assertEqual(res, self.cube1)
    self.assertIsNot(res, self.cube1)
```
or whatever result you feel is appropriate.