Move models and serializers in test to fixtures.rb file

test/test_helper.rb

@@ -1,6 +1,7 @@
 require 'bundler/setup'
 require 'newbase/active_model_serializers'
 require 'test/unit'
+require 'newbase/fixtures'
 
 module TestHelper
   Routes = ActionDispatch::Routing::RouteSet.new