Clean up Timer tasks

src/frequenz/channels/timer.py

@@ -682,14 +682,17 @@ async def ready(self) -> bool:  # noqa: DOC502
         # could be reset while we are sleeping, in which case we need to recalculate
         # the time to the next tick and try again.
         while time_to_next_tick > 0:
-            await next(
-                asyncio.as_completed(
-                    [
-                        asyncio.sleep(time_to_next_tick / 1_000_000),
-                        self._reset_event.wait(),
-                    ]
-                )
-            )
+            tasks = [
+                asyncio.create_task(asyncio.sleep(time_to_next_tick / 1_000_000)),
+                asyncio.create_task(self._reset_event.wait()),
+            ]
+
+            try:
+                await next(asyncio.as_completed(tasks))
+            finally:
+                tasks[0].cancel()
+                tasks[1].cancel()
+
             self._reset_event.clear()
             now = self._now()
             time_to_next_tick = self._next_tick_time - now