flaky tests

e.g. in https://github.com/dandi/backups2datalad/actions/runs/18325676600/job/52189430406?pr=84 for py 3.10 but happened similarly in others AFAIK . So far torturing one in a loop locally as `( set -e ; for s in {1..10}; do BACKUPS2DATALAD_TESTS_PERSIST_DOCKER_COMPOSE=1 tox -e py312 -- -s -v test/test_commands.py::test_backup_command; done; )` did not reproduce so might be some order effects or ... ?

```
FAILED test/test_commands.py::test_backup_command - AssertionError: assert {'.dandi/assets-state.json', '.dandi/assets.json', 'dandiset.yaml'} == {'subdir1/apple.txt', '.dandi/assets-state.json', 'subdir2/banana.txt', 'subdir2/coconut.txt', '.dandi/assets.json', 'dandiset.yaml', 'file.txt', 'v0.txt'}

Extra items in the right set:
'subdir2/coconut.txt'
'v0.txt'
'subdir1/apple.txt'
'subdir2/banana.txt'
'file.txt'

Full diff:
{
'.dandi/assets-state.json',
'.dandi/assets.json',
'dandiset.yaml',
- 'file.txt',
- 'subdir1/apple.txt',
- 'subdir2/banana.txt',
- 'subdir2/coconut.txt',
- 'v0.txt',
}
FAILED test/test_core.py::test_binary - AssertionError: assert {'.dandi/assets-state.json', '.dandi/assets.json', 'dandiset.yaml'} == {'.dandi/assets-state.json', '.dandi/assets.json', 'dandiset.yaml', 'data.dat'}

Extra items in the right set:
'data.dat'

Full diff:
{
'.dandi/assets-state.json',
'.dandi/assets.json',
'dandiset.yaml',
- 'data.dat',
}
```