Git will ignore the content of a folder if the folder doesn’t already contain tracked paths. You know by the presence of test_fold/ that there are files in there (Git will never show you an empty directory as being untracked) and you know that those files are not tracked.
Otherwise, unzipping a folder into your repository could potentially introduce huge amounts of output from git status. The default behavior of simply showing the top-level untracked path makes for much saner output.
If you want Git to show you the content of the folder, use
$ git status -uall
This will show you test_fold/test.txt (and any other files) instead of just test_fold/.