flaky test: unit test file_is_uploaded_to_provided_data_dir

[albinsuresh]

Describe the bug

Flaky unit test: file_transfer_server::http_rest::tests::file_is_uploaded_to_provided_data_dir

Failed instances:

• https://github.com/thin-edge/thin-edge.io/actions/runs/7741737024/job/21109328508?pr=2649
• https://github.com/thin-edge/thin-edge.io/actions/runs/7785110414/job/21226953725

To Reproduce

Random failure in the CI. Not consistently reproducible.

Expected behavior

Always pass.