Move authentication test files generation to test_unit specific generator

[joaquintomas2003]

Motivation / Background

This Pull Request has been created because the authentication generator is currently creating minitest specific files regardless of the testing framework used by the user's application.

This can be replicated by:

1. Creating a new Rails app (skip test files).
2. Add rspec-rails to the app.
3. rails g authentication

This will create test/mailers/previews/passwords_mailer_preview.rb and test/test_helpers/session_test_helper.rb (this last one only if using rails from main), regardless of the test framework being rspec.

Detail

This Pull Request changes the authentication generator, moving the steps that involve creating test related files to the test_unit specific generator.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.