Fix flaky test tests/test_credentials_bulk.py::test_an_operator_deletes_500_matching_rows_in_one_call_with_a_fixed_query_count

The merge lane saw `tests/test_credentials_bulk.py::test_an_operator_deletes_500_matching_rows_in_one_call_with_a_fixed_query_count` fail and then pass on a rerun of the same tree. - Test: `tests/test_credentials_bulk.py::test_an_operator_deletes_500_matching_rows_in_one_call_with_a_fixed_query_count` - Tree: `9491c354868ab15e6d4f1d4f972dff3f49ffe27d` - Lane task: 9753 - Flake record: 01a16ffa-f640-70f6-8e36-ca3fb5aad223 Make it deterministic; do not delete it unless it tests nothing.