Use test-unit-ruby-core instead of deep-copy from ruby repository

Author: hsbt

Gemfile

@@ -7,6 +7,7 @@ group :development do
   gem 'racc', '> 1.4.10'
   gem 'kpeg', '>= 1.3.3'
   gem 'test-unit'
+  gem 'test-unit-ruby-core'
   gem 'rubocop', '>= 1.31.0'
   gem 'gettext'
 end