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
diff --git a/library/Osmocom_VTY_Functions.ttcn b/library/Osmocom_VTY_Functions.ttcn
index cb3434e..e5f615c 100644
--- a/library/Osmocom_VTY_Functions.ttcn
+++ b/library/Osmocom_VTY_Functions.ttcn
@@ -68,7 +68,8 @@
 			[] pt.receive(pattern "[\w-]+\(*\)\# ") { };
 			[] pt.receive(t_vty_unknown) {
 				if (strict) {
-					testcase.stop(fail, "VTY: Unknown Command");
+					setverdict(fail, "VTY: Unknown Command");
+					mtc.stop;
 				} else {
 					log("VTY: Unknown Command (ignored)");
 					buf := buf & rx;