test: synchronize deadline fanout assertion

tests/test_deadline_reminders.py

@@ -238,6 +238,7 @@ async def test_deadline_reminders_bound_fanout_and_isolate_failed_devices(tmp_pa
         )
     active = 0
     peak = 0
+    two_active = asyncio.Event()
 
     async def assigned():
         return {"complete": True, "items": [{"id": 1, "due_date": "2026-08-14T00:00:00Z"}]}
@@ -246,7 +247,9 @@ async def test_deadline_reminders_bound_fanout_and_isolate_failed_devices(tmp_pa
         nonlocal active, peak
         active += 1
         peak = max(peak, active)
-        await asyncio.sleep(0.01)
+        if active == 2:
+            two_active.set()
+        await asyncio.wait_for(two_active.wait(), timeout=0.5)
         active -= 1
         if subscription["endpoint"].endswith("device-1"):
             raise RuntimeError("provider failed")