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.gz

$ git switch reduce-gem-size

$ gem build -o after.tar.gz

du -sh before.tar.gz after.tar.gz
784K	before.tar.gz
 12K	after.tar.gz

Metric	Before	After	Saved
Size	784K	12K	−772K (⏷ −98.46%)

whitch files was deleted?

data
 ├── app
 │   ├── assets
 │   │   └── javascripts
 │   │       ├── local-time.es2017-esm.js
 │   │       └── local-time.es2017-umd.js
 │   └── helpers
 │       └── local_time_helper.rb
 ├── lib
 │   ├── local_time
 │   │   └── version.rb
 │   └── local_time.rb
 ├── MIT-LICENSE
 ├── README.md
-└── test
-    ├── helpers
-    │   └── local_time_helper_test.rb
-    └── javascripts
-        ├── fixtures
-        │   ├── index.html
-        │   └── time_zone_check.html
-        ├── server.mjs
-        ├── src
-        │   ├── format24_test.js
-        │   ├── i18n_test.js
-        │   ├── index.js
-        │   ├── local_time_test.js
-        │   ├── relative_date_test.js
-        │   ├── strftime_test.js
-        │   ├── test_helpers.js
-        │   └── time_ago_test.js
-        └── vendor
-            ├── moment.js
-            └── sinon.js

ps: you can see on rails repo

[yuri-zubov]

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