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
 28K	before.tar
 16K	after.tar

Metric	Before	After	Saved
Size	28K	16K	−12K (⏷ −42.9%)

whitch files was deleted?

data
 ├── lib
 │   ├── rails
 │   │   └── html
 │   │       ├── sanitizer
 │   │       │   └── version.rb
 │   │       ├── sanitizer.rb
 │   │       └── scrubbers.rb
 │   └── rails-html-sanitizer.rb
-├── test
-│   ├── rails_api_test.rb
-│   ├── sanitizer_test.rb
-│   ├── scrubbers_test.rb
-│   └── test_helper.rb
 ├── CHANGELOG.md
 ├── MIT-LICENSE
 └── README.md

ps: you can see on rails repo

[yuri-zubov]

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

[flavorjones]

@yuri-zubov Thank you!