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
 16K 	before
 12K 	after

	before	after	saved
size	16K	12K	-4K (⏷ -25.00%)

whitch files was deleted?

 data
-├── .github
-│   └── workflows
-│       └── testing.yml
-├── .gitignore
-├── active_record_union.gemspec
-├── bin
-│   ├── console
-│   └── create-db-users
-├── Gemfile
 ├── lib
 │   ├── active_record_union
 │   │   ├── active_record
 │   │   │   └── relation
 │   │   │       └── union.rb
 │   │   └── version.rb
 │   └── active_record_union.rb
 ├── LICENSE.txt
-├── rails_6.0.gemfile
-├── rails_6.1.gemfile
-├── rails_7.0.gemfile
-├── rails_7.1.gemfile
-├── rails_7.2.gemfile
-├── rails_8.0.gemfile
-├── Rakefile
 ├── README.md
-└── spec
-    ├── spec_helper.rb
-    ├── support
-    │   ├── databases.rb
-    │   └── models.rb
-    └── union_spec.rb

ps: you can see on rails repo