It's a regression test to make sure that no exception is thrown. Before the addition of the __hash__() method, this is what would happen:

```python
>>> hash(sympy.plotting.intervalmath.interval_arithmetic.interval(1, 2))
Traceback (most recent call last):
  File "<console>", line 1, in <module>
TypeError: unhashable type: 'interval'
>>> type(sympy.plotting.intervalmath.interval_arithmetic.interval(1, 2))
<class 'sympy.plotting.intervalmath.interval_arithmetic.interval'>
```

I suppose I could test individual values, although it doesn't really matter what the values are, I don't think. Just that the objects are hashable.