validation: Fix bad thread dispatch validation

Where the robust thread dispatching is disabled, the
ISRDisabledOnDemandThreadDispatchTask() has to exit.

Update #3716.

testsuites/validation/tc-score-fatal.c

@@ -193,6 +193,7 @@ static void ISRDisabledOnDemandThreadDispatchTask( rtems_task_argument arg )
   rtems_interrupt_local_disable( level );
   (void) level;
   SetSelfPriority( PRIO_VERY_HIGH );
+  rtems_task_exit();
 }
 
 static void FatalBadThreadDispatchDisableLevel(