Fix a false negative for Rails/ActionControllerTestCase when class is namespaced

[vlad-pisanov]

Consider

class MyControllerTest1 < ActionController::TestCase
end

class Foo::Bar::MyControllerTest2 < ActionController::TestCase
end

Currently, Rails/ActionControllerTestCase flags the first class as an offence, but not the second one because it is namespaced. This PR addresses that problem.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

@vlad-pisanov This looks good to me. Can you rebase with the latest master branch?

[koic]

I will close this PR because this patch has been cherry-picked in the master branch. Thank you.
c532db1