Tezt: fix redundancy in error message

Context

When cleaning up temporary directories, Tezt check whether they are empty before removing them. If not, it emits a warning. Currently, this warning looks like:

Directory directory is not empty /path/to/dir

This MR fixes it to make it look like this instead:

Directory is not empty /path/to/dir

Manually testing the MR

Checklist

• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR