Add Support for classes defined with compact style

[sinsoku]

There is a problem that Rails/UniqueValidationWithoutIndex doesn't work
because it can't find the table name in the class defined with compact style.

This commit adds test code to ensure existing behavior and adds support for
classes defined with compact style.

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

Can you rebase with the latest master branch and add a reproduction test case to spec/rubocop/cop/rails/unique_validation_without_index_spec.rb?

[sinsoku]

I rebased and force-pushed.

The test case of "when the class is defined with compact style" is the reproduction test case.
Should I split a commit?

[koic]

I think it's happening the user level issue at Rails/UniqueValidationWithoutIndex cop. spec/rubocop/cop/active_record_helper_spec.rb has written for ActiveRecordHelper internal API test case.
I'm hoping that a test for Rails/UniqueValidationWithoutIndex cop will be added to spec/rubocop/cop/rails/unique_validation_without_index_spec.rb.

[sinsoku]

Thanks for your review comment.
I understood.

I added a test case for Rails/UniqueValidationWithoutIndex and force-pushed it.

[koic]

Nice 👍 Great thanks!