sftp backend test weirdness

It's not visible on github actions CI (we skip the sftp tests there), but the sftp backend / store test sometimes behaves weird:

Especially the `test_scalability_big_values` and the `test_scalability_size` seem to trigger test teardown errors (the `.destroy` is then failing, not cleaning up the sftp server and then causing more errors in the following tests).

When using `pytest -v -rs -k "not big_values and not scalability_size"`, all works fine.

I am seeing this on macOS 15 with the default macOS sftp server on localhost.

Update: when using a remote sftp server (debian linux based, OpenSSH), it works.