test: call "udevadm settle" after stopping array.

If we don't do this, then the unlink from /dev might happen
after the next step in the test creates something in /dev,
and device names seem to go missing.

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

test

@@ -101,6 +101,9 @@ mdadm() {
         * )   $mdadm 2> $targetdir/stderr --quiet "$@"
     esac
     rv=$?
+    case $* in
+       *-S* ) udevadm settle;;
+    esac
     cat >&2 $targetdir/stderr
     return $rv
 }