Remove test files to reduce gem size

[ybiquitous]

Closes #951
Follows up to #771

This aims to reduce the gem package size by removing test files from a built gem.

[Watson1978]

Thanks!

[ybiquitous]

Thanks for the quick review 😄

To resolve the CI failure, it seems necessary to add gem 'logger' to gemfiles/rails_7.gemfile:

/home/runner/work/oj/oj/vendor/bundle/ruby/3.5.0+0/gems/concurrent-ruby-1.3.4/lib/concurrent-ruby/concurrent/concern/logging.rb:1: warning: logger was loaded from the standard library, but is not part of the default gems starting from Ruby 3.5.0.
You can add logger to your Gemfile or gemspec to silence this warning.
/home/runner/.rubies/ruby-head/lib/ruby/3.5.0+0/bundled_gems.rb:76:in 'Kernel.require': cannot load such file -- logger (LoadError)

https://github.com/ohler55/oj/actions/runs/12762636078/job/35575532139?pr=952#step:4:68