test-repart: use cryptsetup and losetup autoclose

test-repart: use cryptsetup and losetup autoclose

The test occasionally fails as the umount is not yet completed when
cryptsetup close is invoked.
Both cryptsetup and losetup have supported deferred cleanup for some
time now, so use it instead to avoid races.

++ losetup -P --show --find /tmp/test-repart.dMOfYQ8UUF/zzz
+ LOOP=/dev/loop6
+ VOLUME=test-repart-11882
+ touch /tmp/test-repart.dMOfYQ8UUF/empty-password
+ cryptsetup open --type=luks2 --key-file=/tmp/test-repart.dMOfYQ8UUF/empty*** test-repart-11882
+ mkdir /tmp/test-repart.dMOfYQ8UUF/mount
+ mount -t ext4 /dev/mapper/test-repart-11882 /tmp/test-repart.dMOfYQ8UUF/mount
+ diff -r /tmp/test-repart.dMOfYQ8UUF/mount/def /tmp/test-repart.dMOfYQ8UUF/definitions
+ umount /tmp/test-repart.dMOfYQ8UUF/mount
+ cryptsetup close test-repart-11882
Device test-repart-11882 is still in use.
+ rm -rf /tmp/test-repart.dMOfYQ8UUF

(cherry picked from commit 0efcbecd8b266526bf4a8fd06b9b7e7c5c43566d)