test: make TEST-64 btrfs_basic cleanup robust against reruns

test: make TEST-64 btrfs_basic cleanup robust against reruns

The LUKS subtest in testcase_btrfs_basic leaves stale LUKS headers on
the underlying SCSI devices, so if the VM is rebooted the test fails
because the LUKS signature is still there and blkid finds it.

[    7.683] + udevadm lock ... mkfs.btrfs -f -L btrfs_root -U deadbeef-dead-dead-beef-000000000000 /dev/disk/by-id/scsi-0systemd_foobar_deadbeefbtrfs0
[    7.729]   Label: btrfs_root
[    7.729]   UUID:  deadbeef-dead-dead-beef-000000000000
[    7.743] + udevadm wait --settle --timeout=30 /dev/disk/by-id/scsi-0systemd_foobar_deadbeefbtrfs0 /dev/disk/by-uuid/deadbeef-dead-dead-beef-000000000000 /dev/disk/by-label/btrfs_root
[    7.788] sda: ... SYMLINK+="disk/by-label/$env{ID_FS_LABEL_ENC}": Added device node symlink "disk/by-label/encdisk0".
[   37.998] Timed out for waiting devices being initialized.
[   38.002] TEST-64-UDEV-STORAGE-btrfs_basic.service: Main process exited, code=exited, status=1/FAILURE

Likewise for the BTRFS UUID:

ERROR: non-unique UUID: deadbeef-dead-dead-beef-000000000001

So wipe that too.