xdg-utils/tests/t.xdg-copy and t.xdg-desktop use files with hard-coded names in
/tmp. That's not very polite...means users can interfere with each others'
tests. At an extreme, it looks like a dangerous race condition...what happens if
one is running the tests as root on a slow system, and someone else manages to
symlink the $dest to /etc/passwd? Better to use 'tempfile -d' to create a
private arena for the tests, or else to use to use the local tests/ directory
(or a subdirectory).