Fix flaky test tests/test_org_setup.py::test_add_slots_ignores_assignments_that_cannot_run_work

The merge lane saw `tests/test_org_setup.py::test_add_slots_ignores_assignments_that_cannot_run_work` fail and then pass on a rerun of the same tree. - Test: `tests/test_org_setup.py::test_add_slots_ignores_assignments_that_cannot_run_work` - Tree: `f6e4d75b9b43a43785939cbd932a381c53a63b4d` - Lane task: 12965 - Flake record: 01a19e97-5058-70ed-8dd9-18ebd76b4321 Make it deterministic; do not delete it unless it tests nothing.

Not a test defect: the lane's "failure" was an xdist worker crash (same signature on unrelated tests since the 3.14t runtime). The test passed 20/20 alone and 91/91 for the file ×4. No change. Filed draft 12986 for the worker crashes. Memory: none applied.