Favor ActiveSupport::TestCase over Minitest::Test

- Better minitest 4/5 support
- Better DSL
- Already available with no changes
- Consistent interface

test/serializers/fieldset_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 module ActiveModel
   class Serializer
-    class FieldsetTest < Minitest::Test
+    class FieldsetTest < ActiveSupport::TestCase
       def test_fieldset_with_hash
         fieldset = ActiveModel::Serializer::Fieldset.new('post' => %w(id title), 'comment' => ['body'])
         expected = { :post => [:id, :title], :comment => [:body] }