add some test cases (work on tmp_folder)

[jgsogo]

Hi! Playing with the tests that you proposed for conan-io#4092 I found one use case that is failing, check this line: https://github.com/efremovd/conan/compare/ftp-download-auth...jgsogo:efremovd-ftp-download-auth?expand=1#diff-3c7937871325b48b30cc01374d2a0cbaR37. Please, consider this PR and let me know if your are going to fix the invalid-file issue or if you want me to propose the change.

Also, I've rewritten the test class to use a temp folder and clean it up when everything is finished.

Thanks!! All contributions are more than welcome 👐

[efremovd]

Hi, thanks :). So what you are saying is that it still creates empty file even though it does not exist on ftp.

I'll fix it later, thank you anyway.

[jgsogo]

So what you are saying is that it still creates empty file even though it does not exist on ftp.

Yes, that is a bug in Conan sources, not related to your issue/fix, but I think it can go in the same PR.

Thanks!