Remove test files from package

[f440]

This pull request updates the *.gemspec file to optimize the gem package size and structure. The changes include:

• Modified files to exclude test files from the package.
• Removed the deprecated test_files specification as it is no longer recommended.

These changes reduce the extracted package size from 2.7MB to 2.2 MB.

Benefits:

• Reduced container sizes when the gem is included
• Shorter download times for developers / CI

References:

• https://github.com/rubygems/guides/issues/90
• https://github.com/rubygems/bundler/pull/3207
• Template for the *.gemspec file generated by bundle gem command