monitor_testcase_8: Wait for sync after re-adding device

Re-adding the device might be triggering a re-synchronization
of the array, which is a different operation as just re-adding
a device. So mdadm --wait might return even if the recovery
is about to start. Hence we need to wait for synchronization, too.

Signed-off-by: Hannes Reinecke <hare@suse.de>