Added an import of `median` from the `filters` module

(it's `filters.rank.median`, but it is convenient to have the very common
median filter in `filters` as well)

Following Issue #1236 

Are you using this to test whether median is indeed in filters? If so, I'd prefer for you to wrap this in a test function, e.g.

```python
def test_median_in_filters():
    from skimage.filters import median
```

This will make it easier to debug.

If it's here for another reason, maybe document it with a comment. =P

Also, maybe not the right test file, since it's there to test for a specific API deprecation, and thus could one day be removed...