Otherwise, this breaks whenever the test directory contains the image
format (e.g. "/tmp/test-raw-file" is filtered to "/tmp/test-IMGFMT-file"
instead of "TEST_DIR").

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20201113211718.261671-3-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>