[Finder] Fixes iterator keys

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6891
| License       | MIT

Previous PR I did on finder introduced a BR break (finder iterator keys must be the pathname of the file, not an incremented index). This PR adds a test to ensure this wont be broken again and fixes the BC break.