US Data Rate Decay testing fails

RWC5020A test 9.4 watches the device as it reduces data rate in the face of no feedback from the network. At the minimum data rate, the device is expected *not* to set ADRack request. The current LMIC leaves it set.

There's a tension here, but I guess we should depend on the network to notice that we've gone to the highest data rate and send us parameters.