Move bundler/setup to Rakefile from spec_helper/test_helper

[pravi]

Many gems do this already, for example puma, ruby-shoulda-context etc. If the bundler/setup require is part of tests, we have to patch it, but having it in Rakefile avoids having to maintain a patch but upstream does not lose out anything as their existing workflow will continue to work.

[utkarsh2102]

Hey,

Using "bundler/setup" is redundant in tests. We're just removing it's usage altogether.
See here: utkarsh2102/rubocop-packaging#18

This cop will be shortly released once I migrate the documentation from packaging-style-guide to the docs/ in rubocop-packaging, which should be happening soon!

Please let me know if you want to propose something different?
If not, do you think we can close this issue here in the favor of utkarsh2102/rubocop-packaging#18 being released soon?

[pravi]

Sounds good.