Free online SFTP servers:
You can find a list of public SFTP servers with free access on SFTP.net website. All of them are read-only.
SFTP server to run on your machine:
For read-write access using your own SFTP server would be needed. SFTP.net includes also list of SFTP servers (mostly) for Windows.
Rebex Tiny SFTP Server
But don't be afraid. Two of listed servers are quite minimalistic - Rebex Tiny SFTP server requires no installation at all. Just unpack the ZIP, run the application and push the start button. You can even download server C# source code if you want to modify it for your testing.
Full featured, scriptable SFTP server. No installation needed. Just
download it from BuruServer.com, unzip and run following commands:
buru user add myuser --password mypassword --rootdir c:\data
BuruServer.com - Getting started
BuruServer.com - Downloads
Core Free Mini SFTP Server
Core Free Mini SFTP server setup is also pretty easy. For Core Free Mini server download scroll down to the middle of page. On the top there are downloads for their full-featured server.