Restore test-local subscriber teardown

This commit revises 0ce4ad35a12ffd858f6a5d7bbeb48fc1e2cfac92
`Remove unused/unusable unsubscribe since we don't want to unsubscribe`

Looking at Rails implementation of assert_template
which was likely the inspiration for assert_serializer:

f756b33c13/lib/rails/controller/testing/template_assertions.rb

Ref:

- https://github.com/rails-api/active_model_serializers/pull/596
- https://github.com/rails-api/active_model_serializers/pull/620
- https://github.com/rails-api/active_model_serializers/issues/616

lib/active_model_serializers/test/serializer.rb

@@ -6,6 +6,7 @@ module ActiveModelSerializers
 
       included do
         setup :setup_serialization_subscriptions
+        teardown :teardown_serialization_subscriptions
       end
 
       # Asserts that the request was rendered with the appropriate serializers.
@@ -41,6 +42,7 @@ module ActiveModelSerializers
 
         def initialize
           @serializers = Set.new
+          @_subscribers = []
         end
 
         def message=(message)
@@ -62,12 +64,18 @@ module ActiveModelSerializers
         end
 
         def subscribe
-          ActiveSupport::Notifications.subscribe(event_name) do |_name, _start, _finish, _id, payload|
+          @_subscribers << ActiveSupport::Notifications.subscribe(event_name) do |_name, _start, _finish, _id, payload|
             serializer = payload[:serializer].name
             serializers << serializer
           end
         end
 
+        def unsubscribe
+          @_subscribers.each do |subscriber|
+            ActiveSupport::Notifications.unsubscribe(subscriber)
+          end
+        end
+
         private
 
         def matches_class?
@@ -108,6 +116,10 @@ module ActiveModelSerializers
         @assert_serializer = AssertSerializer.new
         @assert_serializer.subscribe
       end
+
+      def teardown_serialization_subscriptions
+        @assert_serializer.unsubscribe
+      end
     end
   end
 end