Description: Starts or stops (aborts) a test in progress. The agent will
only start a new test, if no test is active on the targeted
port.  The user must set lineTestTimeIntvl to an appropriate
value before starting test by setting this parameter to start.
The System resets the value back to stop when the test
completes.  This parameter value affects lineTestStatus as
following:

     value   current lineTestStatus  new lineTestStatus
     -----   ----------------------  ------------------
     start   inactive/pass/fail/     active               
             aborted
     stop    active                  aborted
     clear   pass/fail/aborted       inactive
    
The rest of the combinations will result in error. 

Starting of the test may fail due to a line test already in
progress on the selected subscriber line.
      
Agent updates lineTestStartTime, when it accepts the line test.
Agent updates lineTestCmplTime, when the test completes or when
user aborts the test.
  
Stopping a test resets lineTestUpBitErrRate and
lineTestDwnBitErrRate.

Clearing a test resets lineTestStartTime, lineTestCmplTime,
lineTestUpBitErrRate, and lineTestDwnBitErrRate to zero.
lineTestTimeIntvl and lineTestBitErrRateLimit get set back to
their defaults.  Finally, lineTestStatus is set to inactive.

Line test information only resets when the user sets this
parameter to clear, otherwise parameters from the previous run
of the test are preserved until system is rebooted. All of the
parameters are initialized on reboot.