generic-worker: add tests for mounting indexed artifacts

[petemoore]

This is a follow-up from #6246 (comment)

[petemoore]

Note, this is just an initial indexed artifact test. It could be expanded in future to cover more test cases:

1. that when the index is updated (new task ID) for an artifact that has already been cached, and then a task references it, that the new artifact is downloaded
2. that when the index is not updated for an artifact that has already been cached, or is updated with e.g. a higher rank but the task ID remains the same, and a new task references the indexed artifact, that the cached artifact is used

[matt-boris]

I like it Pete, thanks! Only tiny nit from me is variable naming. Using payload is a bit confusing since we generally refer to payloads as the task payload. So it hurts readability just a tiny bit. Instead, maybe we could use itr for InsertTaskRequest? Wdyt?