test/driver: fix test case unpacking on macOS
test/driver: fix test case unpacking on macOS

This got through CI because CI was only unpacking a frozen archive, and
not unpacking any tests from scratch.

Fixes: c82e09ac6f ("test/driver: unpack xdelta onto reflinked base file, if possible")
Signed-off-by: Benjamin Gilbert <55d1d0083477f6dd5757d8929dd3b9e09d80bd84@cs.cmu.edu>

openslide/openslide,openslide/openslide,openslide/openslide,openslide/openslide