CLI boundary tests

some more tests for issue #981
- subnet CRUD
- domain CRUD
Please use the `format` method when interpolating data into strings. It is preferred over the `%s` method. If you're wondering why, just ask. I can find the exact reference in the python docs.

Also, while we're at it, might this be more elegant?

```python
bug_id = options.pop('bz-bug', None)
if bug_id is not None and bz_bug_is_open(bug_id):
    self.skipTest('Bugzilla bug {0} is open.'.format(bug_id))
```