Removed unsubscribe as it's not strictly necessary for tests. We modify global state in the spec, but we can prefix the notification names with the test name and I think having a more restricted interface is more valuable than properly cleaning up in a test.

Another argument for not providing unsubscribe is its [poor performance with our current ActiveSupport implementation](api.rubyonrails.org/v3.2.17/classes/ActiveSupport/Notifications.html):
> WARNING: The instrumentation framework is designed for long-running subscribers, use this feature sparingly because it wipes some internal caches and that has a negative impact on performance.