test-kpartx: add test for invocation on image file

test-kpartx: add test for invocation on image file

Add a test for kpartx invocation on an image file that is not
yet part of a loop device, passing a relative path to kpartx:

        # kpartx -av disk.img
        ...
        # kpartx -d disk.img

The recent patches 95c4189b "kpartx: Use absolute paths to create mappings"
and c1adcc5b "kpartx: Use absolute paths to create mappings" both
independently fixed this behavior, which had been broken previously by
9bdfa3eb "kpartx: use absolute path for regular files".

Cc: Stef Walter <stefw@redhat.com>
Cc: Yuval Turgeman <yturgema@redhat.com>
Signed-off-by: Martin Wilck <mwilck@suse.com>