fix the notification_forever_unsubscribed scenario

[pjenvey]

This scenario is currently no different than notification_forever. It's supposed to unregister the client before beginning the forever loop, but due to a bug it doesn't actually do so (it's missing a yield statement):

https://github.com/mozilla-services/ap-loadtester/blob/6a769/aplt/scenarios.py#L194

I don't understand the intention of the scenario -- if the client's unregistered it won't be accepting the notifications later in the loop. The scenario should be rethought or removed

[rpappalax]

we have several scenarios that test bad client behavior (including this one). sounds like the way forward is to ensure that the server is returning a 404. the load generated by misbehaving client is still valid for generating load to stress the server so we probably should adapt this way it if we can.

[pjenvey]

IIRC this one should return 410s. This scenario's definitely worth having (even though nginix handles most of these) and shouldn't be removed (should be fixed)