add test for exploding cases xml

@emord I'm just creating this so I can leave comments. Once we're done, you can go ahead and

```bash
git checkout case-explode-attachments
git pull origin case-explode-attachments-tests
```

This line I'm not so sure about. Can you replace it with how it looked in the example you were using to test? I made this case_attachments dict by printing out the interim state of one of these from a casexml.apps.case test.