tests: don't choke on deleted losetup paths

tests: don't choke on deleted losetup paths

If a file has been deleted with a loopback device attached, then the
`losetup --all` output will carry:
/dev/loopX: [0032]:344213 (/.../src/test-ceph-disk/vdf.disk (deleted))

This causes the losetup parsing in reset_leftover_dev() to throw an
error, e.g.:
rreset_leftover_dev: 430: test
'(/home/ddiss/ceph/src/test-ceph-disk/vdf.disk' '(deleted))' =
'(/home/ddiss/ceph/src/test-ceph-disk/vdf.disk)'
test/ceph-disk.sh: line 430: test: too many arguments

Fix this by quoting the path variable for the string comparison.

Signed-off-by: David Disseldorp <ddiss@suse.de>