I couldn't find a clean way to do it that didn't build a bunch of helper-specific behaviour into the various test classes.

I suppose some kind of `.get_helper(helper_class, *args, **kw)` method that constructed the helper and added its cleanup would be a good things to have. That drops two lines of boilerplate down to one (or three to two in this case) at the cost of making sure all helper classes have a `.cleanup()` method which is fairly sensible anyway.

I'll have to think about ways to get rid of the `.setup_vumi_api()` call. That's tricky because (a) it's potentially async and (b) it's not always desired.