Support :: prefixed constants on some cops

[r7kamura]

After the following Pull Request was merged, I checked to see if any other cop has a similar problem.

• Support ::Rails and ::File on Rails/FilePath cop #865

As a result, I found those problems in the following cops, so I created this Pull Request.

• Rails/ActionControllerFlashBeforeRender
• Rails/ActionControllerTestCase
• Rails/ApplicationController
• Rails/ApplicationJob
• Rails/ApplicationMailer
• Rails/ApplicationRecord
• Rails/DotSeparatedKeys
• Rails/DynamicFindBy
• Rails/FindEach
• Rails/FreezeTime
• Rails/HasManyOrHasOneDependent
• Rails/HelperInstanceVariable
• Rails/MailerName
• Rails/MigrationClassName
• Rails/Output
• Rails/ReversibleMigrationMethodDefinition
• Rails/ReversibleMigration
• Rails/ShortI18n
• Rails/SkipsModelValidations
• Rails/TimeZoneAssignment

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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]

Great 👍 Thank you!