test: fix flaky `test_request_queue_unlock_requests` (#786)

## Summary

Fix [this flaky CI
run](https://github.com/apify/apify-client-python/actions/runs/25495772865/job/74814967391?pr=742)
of `test_request_queue_unlock_requests[sync]`, which failed with `assert
2 == 3` on `unlock_response.unlocked_count`.

**Root cause:** the polling loop called `list_and_lock_head` repeatedly,
which has the side effect of locking items. Across multiple iterations
(due to eventual consistency on `add_request`) the total set of
actually-locked requests no longer matched the last response's `items`
count, so `unlock_requests` could report a different count than the test
expected.

**Fix:** separate the two concerns — poll the read-only `list_head`
until all 5 added requests are visible, then call
`list_and_lock_head(limit=3)` exactly once. This removes the
cross-iteration locking ambiguity.

Applied the same treatment to `test_request_queue_list_and_lock_head`,
which shared the same racy polling pattern.