A disposable SFTP server to test against. Verify that your client connects, that your script uploads, or that your integration writes the file it claims to — then let the whole thing delete itself an hour later.
Every server arrives with accounts.csv, customers.json and
orders.xml already on it, so you can test reading before you have anything
to write.
Write a file, then list the directory and read it back. That round trip is the thing worth testing: plenty of clients report a successful upload while writing to a different directory than you intended.
Have your flow read the sample CSV, transform it, and write the result back; then download the result and check it against what you expected.