Fix test async errors?

[jrdh]

Sometimes the tests output uncollected async task errors from the syncing code, e.g.: https://github.com/NaturalHistoryMuseum/splitgill/actions/runs/15056759969/job/42324210892. I thought I'd fixed this with this: 924b399 but apparently not?

It would be good to get to the bottom of it and make sure it's not an actual error.