Fix a flaky test

This test passes sometimes and fails sometimes: <paste the test and
the code under test>

Find the source of the non-determinism:
- Timing — a fixed wait instead of waiting for a condition.
- Shared state between tests, or tests depending on run order.
- Real dates, times or randomness.
- Real network calls.
- Database state not reset between runs.
- Parallel tests touching the same records.

Fix it properly. Do not add a retry or increase a timeout to hide it —
if the code under test has a real race, I want to know that instead.