Initialize the DUT.

        DUT will be restarted. and openthread will started.

def _init_dut(self):
        """Initialize the DUT.

        DUT will be restarted. and openthread will started.
        """
        if self.auto_dut:
            self.dut = None
            return

        dut_port = settings.DUT_DEVICE[0]
        dut = OpenThreadController(dut_port)
        self.dut = dut