Reduce gem size by excluding test files

[yuri-zubov]

This pull request updates the *.gemspec file to optimize the gem package size and structure

$ gem build -o before.tar

$ git switch reduce-gem-size

$ gem build -o after.tar

$ du -sh before.tar after.tar
 32K	before.tar
 20K	after.tar

Metric	Before	After	Saved
Size	32K	20K	−12K (⏷ −37.5%)

whitch files was deleted?

data
-├── .devcontainer
-│   ├── devcontainer-lock.json
-│   └── devcontainer.json
-├── .github
-│   └── workflows
-│       └── ruby.yml
-├── bin
-│   ├── release
-│   └── test
-├── gemfiles
-│   ├── rails_7_0.gemfile
-│   ├── rails_7_1.gemfile
-│   ├── rails_7_2.gemfile
-│   ├── rails_8_0.gemfile
-│   ├── rails_8_1.gemfile
-│   └── rails_head.gemfile
 ├── lib
 │   ├── generators
 │   │   └── rails
 │   │       ├── templates
 │   │       │   ├── api_controller.rb
 │   │       │   ├── controller.rb
 │   │       │   ├── index.json.jbuilder
 │   │       │   ├── partial.json.jbuilder
 │   │       │   └── show.json.jbuilder
 │   │       ├── jbuilder_generator.rb
 │   │       └── scaffold_controller_generator.rb
 │   ├── jbuilder
 │   │   ├── blank.rb
 │   │   ├── collection_renderer.rb
 │   │   ├── errors.rb
 │   │   ├── jbuilder_dependency_tracker.rb
 │   │   ├── jbuilder_template.rb
 │   │   ├── jbuilder.rb
 │   │   ├── key_formatter.rb
 │   │   ├── railtie.rb
 │   │   └── version.rb
 │   └── jbuilder.rb
-├── test
-│   ├── jbuilder_dependency_tracker_test.rb
-│   ├── jbuilder_generator_test.rb
-│   ├── jbuilder_template_test.rb
-│   ├── jbuilder_test.rb
-│   ├── scaffold_api_controller_generator_test.rb
-│   ├── scaffold_controller_generator_test.rb
-│   └── test_helper.rb
-├── .gitignore
-├── Appraisals
-├── CONTRIBUTING.md
-├── Gemfile
-├── jbuilder.gemspec
 ├── MIT-LICENSE
-├── Rakefile
 └── README.md

ps: you can see on rails repo

[yuri-zubov]

The test_files directive is no longer used by RubyGems.org.