validation: Fix CallWithinISR()

Some BSPs require that Clear_tm27_intr() is called in the interrupt
service routine.

Update #3269.

testsuites/validation/tx-call-within-isr.c

@@ -71,6 +71,8 @@ static void CallWithinISRHandler( rtems_vector_number vector )
   (void) vector;
   ctx = &CallWithinISRInstance;
 
+  Clear_tm27_intr();
+
   while ( true ) {
     rtems_interrupt_lock_context lock_context;
     CallWithinISRRequest        *request;