Favor ActiveSupport::TestCase over Minitest::Test

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

test/adapter/null_test.rb

@@ -3,7 +3,7 @@ require 'test_helper'
 module ActiveModel
   class Serializer
     module Adapter
-      class NullTest < Minitest::Test
+      class NullTest < ActiveSupport::TestCase
         def setup
           profile = Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' })
           serializer = ProfileSerializer.new(profile)