[Fix #8372] Fix Lint/RedundantCopDisableDirective autocorrection removing a preceding newline incorrectly

[dvandersluis]

First fix for #8372 (I split it into two PRs since it touches two cops), fixes Lint/RedundantCopDisableDirective autocorrection so that it doesn't clobber blank lines. There were no tests previously for autocorrection, so this change also adds tests to ensure whitespace is managed correctly.

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• 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.

[bbatsov]

Thanks!