fix: Exclude tests and related files from gem package

[amatsuda]

Here's a patch that excludes files under spec, test, and gemfiles directory from the gem package.

With this patch, the gem package size shrinks as follows:

before: 246272 bytes
after:  229376 bytes

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [ ] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [ ] I have added inline documentation to the code I modified