test: clear out old metadata from loop devices.

Old metadata can tempt udev to assemble things, which
just gets in the way.

Signed-off-by: NeilBrown <neilb@suse.de>

test

@@ -111,6 +111,8 @@ do_setup() {
     case $DEVTYPE in
     loop)
       [ -f $targetdir/mdtest$d ] || dd if=/dev/zero of=$targetdir/mdtest$d count=$sz bs=1K > /dev/null 2>&1
+      # make sure udev doesn't touch
+      mdadm --zero $targetdir/mdtest$d 2> /dev/null
       [ -b /dev/loop$d ] || mknod /dev/loop$d b 7 $d
       if [ $d -eq 7 ]
       then