abstract some methods in contents service tests

should allow re-use for ContentsManager subclasses

closes #7190

I don't have a ContentsManager to verify this against, so I'd like to hear from @ssanderson on this one, and we can iron out details.
My subclass implementation of this is just:
```
def make_dir(self, api_path):
    self.contents_manager.new(
        model={'type': 'directory'},
        path=api_path,
    )
```
Is there any reason not to just put that in the superclass?