Restore test-local subscriber teardown
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:

https://github.com/rails/rails-controller-testing/blob/f756b33c138c593eabe37f6085f8bac477b99bfe/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

beauby/active_model_serializers,groyoh/active_model_serializers,CorainChicago/active_model_serializers,dpdawson/active_model_serializers,dpdawson/active_model_serializers,nurey/active_model_serializers,beauby/active_model_serializers,domitian/active_model_serializers,groyoh/active_model_serializers,Updater/active_model_serializers,NullVoxPopuli/active_model_serializers,fabianoleittes/active_model_serializers,tarzan/active_model_serializers,ianks/active_model_serializers,AutoCloud/active_model_serializers,nurey/active_model_serializers,domitian/active_model_serializers,NullVoxPopuli/active_model_serializers,remear/active_model_serializers,remear/active_model_serializers,ScottKbka/active_model_serializers,leandrocp/active_model_serializers,Updater/active_model_serializers,Eric-Guo/active_model_serializers,ScottKbka/active_model_serializers,ScottKbka/active_model_serializers,CorainChicago/active_model_serializers,arkiver/active_model_serializers,leandrocp/active_model_serializers,arkiver/active_model_serializers,NullVoxPopuli/active_model_serializers,Updater/active_model_serializers,remear/active_model_serializers,tarzan/active_model_serializers,beauby/active_model_serializers,groyoh/active_model_serializers,Eric-Guo/active_model_serializers,Eric-Guo/active_model_serializers,tarzan/active_model_serializers,arkiver/active_model_serializers,cassidycodes/active_model_serializers,ianks/active_model_serializers,nurey/active_model_serializers,dpdawson/active_model_serializers,rails-api/active_model_serializers,AutoCloud/active_model_serializers