fakelb: add xmit_async after stop testcase

This patch adds a suspended testcase into the xmit_async functionality.
In the hope that we can found race conditions when xmit_async is called
after an ieee802154_ops stop. This should never happen.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>