This attribute is the type of test to be run. The available test types
include the following: card test sets up an internal loop on the board
and runs a bit error rate test; manual test assumes the line is looped
back externally and a bit error rate test is performed; localLoop test
loops back the local modem and performs a bit error rate test;
remoteLoop test loops back the remote modem (or CSU) and
performs a bit error rate test; externalLoop test loops back the line
to the far end; payloadLoop test only loops back the payload data
received from the incoming line; Note that localLoop, remoteLoop
and payloadLoop are not supported by all function processors.