test: make TEST-64 mdadm_lvm cleanup robust against reruns

test: make TEST-64 mdadm_lvm cleanup robust against reruns

mdadm --zero-superblock only wipes the MD metadata on the underlying
disks, not the LVM PV header that lives in the array data area. When
the VM is restarted and the test re-creates the array with the same
UUID, /dev/md127 exposes the old data including the LVM PV header, so
udev's 69-lvm.rules auto-triggers lvm-activate-mdlvm_vg.service which
races with the test's own pvcreate for exclusive access on /dev/md127.

Wipe the LVM signature off the MD device (and the underlying disks as
a belt-and-braces measure) to avoid the race on re-run, fixing failures
when the VM is rebooted instead of shut down.

Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>