[tests] use dedicated _expect_done method for success commands (#6073)
[tests] use dedicated _expect_done method for success commands (#6073)

We see self._expect('Done') in many places, looks like it's worth a
dedicated method.
jwhui/openthread,abtink/openthread,abtink/openthread,srickardti/openthread,openthread/openthread,jwhui/openthread,openthread/openthread,srickardti/openthread,abtink/openthread,srickardti/openthread,jwhui/openthread,openthread/openthread,openthread/openthread,abtink/openthread,srickardti/openthread,jwhui/openthread