Automate testing with multiple client implementations

[koenw]

The current integration tests all use the same FTP client implementation (rust-ftp). By testing (automatically) with a greater range of clients we can be more confident in real-world performance.

I was thinking ftp from GNU inetutils should be easy to script, as should be some of the scripting language's FTP client APIs. It would be great if one of the popular graphical clients is scriptable.

We also need to arrange for something like docker or Vagrant to make this manageable.