Favor ActiveSupport::TestCase over Minitest::Test

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

test/adapter/json_api/pagination_links_test.rb

@@ -8,7 +8,7 @@ module ActiveModel
   class Serializer
     module Adapter
       class JsonApi
-        class PaginationLinksTest < Minitest::Test
+        class PaginationLinksTest < ActiveSupport::TestCase
           URI = 'http://example.com'
 
           def setup