add UNIT_TEST=true into azure

Author: KazuCocoa

azure-pipelines.yml

@@ -19,5 +19,5 @@ steps:
 - script: gem uninstall --force eventmachine && gem install eventmachine --platform ruby
   displayName: 'bundle re-install eventmachine for Windows because of Windows environment issue'
 
-- script: bundle exec parallel_test test/unit/ -n 4
+- script: UNIT_TEST=true bundle exec parallel_test test/unit/ -n 4
   displayName: 'bundle exec parallel_test test/unit/ -n 4'