Fix flaky test tests/billing/test_subscriptions.py::test_the_remaining_r10_rows

The merge lane saw `tests/billing/test_subscriptions.py::test_the_remaining_r10_rows` fail and then pass on a rerun of the same tree. - Test: `tests/billing/test_subscriptions.py::test_the_remaining_r10_rows` - Tree: `d4c6bde9e5ef0917730a9004373af2e8e011c698` - Lane task: 13310 - Flake record: 01a1a250-f342-78fc-bd22-b0a3c2bd80c7 Make it deterministic; do not delete it unless it tests nothing.

Not a flaky test: its xdist worker died of SIGSEGV in native code (no OOM, no dump). The test passed 30/30 isolated and 3/3 full-file at -n 8. No change made; root cause filed as draft Task 13326. Memory: none applied.