[Fix #4967] Fix Rails/DynamicFindBy false-positives on non ActiveReord

[koic]

Fixes #4967.
This PR applies Rails/DynamicFindBy cop only to receiver classes that inherit ActiveRecord::Base class.

---

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).
• Used the same coding conventions as the rest of the project.
• 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.
• All tests(rake spec) are passing.
• The new code doesn't generate RuboCop offenses that are checked by rake internal_investigation.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[Drenmi]

Unfortunately this will not work, because it is based on runtime analysis. 😕

It incidentally works in the tests, because you define the constants in a before block, but if you try to inspect a file with the same code, you'll most likely get an "uninitialized constant" error, because RuboCop doesn't load your codebase.

[koic]

Oh, It's exactly as you said. I overlooked important things 💦 I will close this PR because it requires a different approach to solving this issue. Thank you for the review.