[thci] convert MLR string to PlatfromDiagnosticsPacket (#219)
[thci] convert MLR string to PlatfromDiagnosticsPacket (#219)

Thread Harness expects the MLR log in PlatfromDiagnosticsPacket class.
openthread/ot-commissioner,openthread/ot-commissioner,openthread/ot-commissioner,openthread/ot-commissioner