Avoid lingering tasks when using background backup tasks (#5518)

When a backup tasks is run in background, but actually has an error
early the secondary event task to release the callee is lingering around
still, ultimately leading to a "Task was destroyed but it is pending!"
asyncio error.

Make sure we cancel the event task in case the backup returns early.

supervisor/api/backups.py

@@ -296,13 +296,18 @@ async def release_on_freeze(new_state: CoreState):
             BusEvent.SUPERVISOR_STATE_CHANGE, release_on_freeze
         )
         try:
-            await asyncio.wait(
+            event_task = self.sys_create_task(event.wait())
+            _, pending = await asyncio.wait(
                 (
                     backup_task,
-                    self.sys_create_task(event.wait()),
+                    event_task,
                 ),
                 return_when=asyncio.FIRST_COMPLETED,
             )
+            # It seems backup returned early (error or something), make sure to cancel
+            # the event task to avoid "Task was destroyed but it is pending!" errors.
+            if event_task in pending:
+                event_task.cancel()
             return (backup_task, job.uuid)
         finally:
             self.sys_bus.remove_listener(listener)