Make test attributes explicit

- Organize test poros with associations and by serializer
- Freeze derived attributes/associations against mutation
- Cleanup PORO fixtures

test/serializers/associations_test.rb

@@ -8,7 +8,7 @@ module ActiveModel
         @author.roles = []
         @blog = Blog.new(name: 'AMS Blog')
         @post = Post.new(title: 'New Post', body: 'Body')
-        @tag = Tag.new(name: '#hashtagged')
+        @tag = Tag.new(id: 'tagid', name: '#hashtagged')
         @comment = Comment.new(id: 1, body: 'ZOMG A COMMENT')
         @post.comments = [@comment]
         @post.tags = [@tag]
@@ -53,7 +53,7 @@ module ActiveModel
 
           assert_equal :tags, key
           assert_nil serializer
-          assert_equal [{ name: '#hashtagged' }].to_json, options[:virtual_value].to_json
+          assert_equal [{ id: 'tagid', name: '#hashtagged' }].to_json, options[:virtual_value].to_json
         end
       end
 
@@ -62,7 +62,13 @@ module ActiveModel
                       .associations
                       .detect { |assoc| assoc.key == :comments }
 
-        assert association.serializer.first.custom_options[:custom_options]
+        comment_serializer = association.serializer.first
+        class << comment_serializer
+          def custom_options
+            instance_options
+          end
+        end
+        assert comment_serializer.custom_options.fetch(:custom_options)
       end
 
       def test_belongs_to
@@ -159,7 +165,9 @@ module ActiveModel
 
       class NamespacedResourcesTest < ActiveSupport::TestCase
         class ResourceNamespace
-          class Post    < ::Model; end
+          class Post < ::Model
+            associations :comments, :author, :description
+          end
           class Comment < ::Model; end
           class Author  < ::Model; end
           class Description < ::Model; end
@@ -200,7 +208,9 @@ module ActiveModel
       end
 
       class NestedSerializersTest < ActiveSupport::TestCase
-        class Post    < ::Model; end
+        class Post < ::Model
+          associations :comments, :author, :description
+        end
         class Comment < ::Model; end
         class Author  < ::Model; end
         class Description < ::Model; end
@@ -240,7 +250,10 @@ module ActiveModel
 
         # rubocop:disable Metrics/AbcSize
         def test_conditional_associations
-          model = ::Model.new(true: true, false: false)
+          model = Class.new(::Model) do
+            attributes :true, :false
+            associations :association
+          end.new(true: true, false: false)
 
           scenarios = [
             { options: { if:     :true  }, included: true  },