test: Fix flaky `*_collection_iterate` tests with polling helper (#831)

## Summary

Tests are still flaky due to eventual consistency - newly created
storages aren't immediately visible in the listing endpoint. Adding a
polling helper.

Tested with 50 runs, no flakiness.

## Failing CI run

-
https://github.com/apify/apify-client-python/actions/runs/26557636852/job/78232781171