log: for VTY Unknown Command, fix weird log message

In f_vty_wait_for_prompt(), this:

   testcase.stop(fail, "VTY: Unknown Command")

outputs:

   failVTY: Unknown Command

The first stop() argument 'fail' is not actually a test verdict, but gets
rolled into a log output string, becoming "failVTY".

Fix that by simply using the normal pattern of setverdict() followed by
mtc.stop().

Change-Id: Id09986444de02c10b4fba582d454d54568b6e8a2
1 file changed