Add tests.

test/serializers/inline_serializers_test.rb

@@ -0,0 +1,25 @@
+module ActiveModel
+  class Serializer
+    class InlineSerializersTest < Minitest::Test
+      class PostSerializer < ActiveModel::Serializer
+        attributes :title, :body
+        belongs_to :author
+        has_many :comments do
+          attributes :body
+          belongs_to :author
+        end
+      end
+
+      def test_inline_serializer_defined_if_block_given
+        refute_nil("#{self.class}::PostSerializer::CommentSerializer".safe_constantize)
+        assert_equal(ActiveModel::Serializer, PostSerializer::CommentSerializer.superclass)
+        assert_equal([:author], PostSerializer::CommentSerializer._reflections.map(&:name))
+        assert_equal([:body], PostSerializer::CommentSerializer._attributes)
+      end
+
+      def test_inline_serializer_not_defined_unless_block_given
+        assert_nil("#{self.class}::PostSerializer::AuthorSerializer".safe_constantize)
+      end
+    end
+  end
+end