Move serializer tests into their own directory.

It'd be nice to keep some hygene here.

test/serializers/attributes_test.rb

@@ -0,0 +1,18 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class AttributesTest < Minitest::Test
+      def setup
+        @profile = Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' })
+        @profile_serializer = ProfileSerializer.new(@profile)
+      end
+
+      def test_attributes_definition
+        assert_equal([:name, :description],
+                     @profile_serializer.class._attributes)
+      end
+    end
+  end
+end
+