Sign-out current user when calling `sign_in` in tests and fix an example that only passes on retry

What does this MR do?

1. This monkey-patch Devise::Test::ControllerHelpers#sign_in to clear out the :"@current_#{scope}" controller instance variable so that the previously logged-in user isn't reused upon retries (when using rspec-retry).
   • I raised the issue upstream here: https://github.com/heartcombo/devise/issues/5190.
2. This also fix a test that was only passing on retry due to the above bug (i.e. when only the Devise::Test::ControllerHelpers#sign_in bug is fixed, the test doesn't pass, even on retries).
   • The permission was changed with 6d7002d0 but due to the above bug, the test would still pass upon retries.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Closes #202072 (closed).