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

$ git switch reduce-gem-size

$ gem build -o after

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

	before	after	saved
size	32K	20K	-12K (⏷ -37.50%)

whitch files was deleted?

 data
-├── .github
-│   └── workflows
-│       └── CI.yml
-├── .gitignore
-├── attr_encrypted.gemspec
 ├── CHANGELOG.md
-├── checksum
-│   ├── attr_encrypted-3.0.0.gem.sha256
-│   ├── attr_encrypted-3.0.0.gem.sha512
-│   ├── attr_encrypted-3.0.1.gem.sha256
-│   ├── attr_encrypted-3.0.1.gem.sha512
-│   ├── attr_encrypted-3.0.2.gem.sha256
-│   ├── attr_encrypted-3.0.2.gem.sha512
-│   ├── attr_encrypted-3.0.3.gem.sha256
-│   ├── attr_encrypted-3.0.3.gem.sha512
-│   ├── attr_encrypted-3.1.0.gem.sha256
-│   └── attr_encrypted-3.1.0.gem.sha512
-├── Gemfile
 ├── lib
 │   ├── attr_encrypted
 │   │   ├── adapters
 │   │   │   ├── active_record.rb
 │   │   │   └── sequel.rb
 │   │   └── version.rb
 │   └── attr_encrypted.rb
 ├── MIT-LICENSE
-├── Rakefile
 ├── README.md
-└── test
-   ├── active_record_test.rb
-    ├── attr_encrypted_test.rb
-    ├── compatibility_test.rb
-    ├── legacy_active_record_test.rb
-    ├── legacy_attr_encrypted_test.rb
-    ├── legacy_compatibility_test.rb
-    ├── legacy_sequel_test.rb
-    ├── run.sh
-    ├── sequel_test.rb
-    └── test_helper.rb
    

ps: you can see on rails repo

[yuri-zubov]

@joshbranham It wasn't merge in auto-merge mode