Avoid erroring if _subscribers undefined

It's possible for tests to error during setup before setup_subscriptions
has run. In that case we should avoid erroring in teardown to avoid
confusing the source of the issue.

lib/rails/controller/testing/template_assertions.rb

@@ -51,6 +51,7 @@ def setup_subscriptions
         end
 
         def teardown_subscriptions
+          return unless defined?(@_subscribers)
           @_subscribers.each do |subscriber|
             ActiveSupport::Notifications.unsubscribe(subscriber)
           end