Drop 'return' statements after 'mtc.stop' as they cause following
warning from TTCN-3 compiler:
warning: Control never reaches this statement
Change-Id: I6210ecf5fcb39f751116ad63a69d2ae8651a60c5
This will prevent subsequent failures from overwriting the verdict so we
can easily see the root cause of the test failure.
Using testcase.stop instead for errors internal to our test
infrastructure to mark them as test errors instead of failed.
Change-Id: Idc6819aaf0b01e70c38fad828dd44dcec6bdd778