The Python language makes no guarantee that this will work. So at a minimum this needs a `self.assertIsNot(calendar, iris.unit.CALENDAR_GREGORIAN)` before continuing with the rest of the test.

It'd be nice though the assertion above was combined with a more reliable technique. For example:
```python
class MyStr(str):
    pass

calendar = MyStr(iris.unit.CALENDAR_GREGORIAN)
```