Test explicit tracking of unsubscribing

Problem/Motivation

Simplenews tracks explicit unsubscribing, distinguishing it from merely never having subscribed.

This is useful data when programatically subscribing people; it is not unreasonable to want to default to adding people to a list in response to their behavior, but to allow them to opt-out in the future and to respect past opt-outs. Simplenews bulk import functionality currently uses this to (optionally) not resubscribe people who have previously opted out when importing.

However, it would be easy for subscription forms to break this functionality and we lack explicit test coverage of it.

Proposed resolution

Add explicit test coverage that subscription forms do not create explicit unsubscribes when users do not explicitly subscribe to every visible newsletter.