vty: tests: Verify incomplete optional parameters are passed to vty funcs

The test shows that in the case were "single0 on" is executed, VTY
function should return complete "single0 one" but it doesn't.

Related: OS#4045
Change-Id: Ib5b9dc07e2b280dc95011b3926afb1d490cadd81
diff --git a/tests/vty/vty_transcript_test.vty b/tests/vty/vty_transcript_test.vty
index 5ed3c1d..1557744 100644
--- a/tests/vty/vty_transcript_test.vty
+++ b/tests/vty/vty_transcript_test.vty
@@ -75,3 +75,12 @@
 
 vty_transcript_test> multi2 t
 % Ambiguous command.
+
+vty_transcript_test> single0 one
+ok argc=1 one
+
+vty_transcript_test> single0 on
+ok argc=1 on
+
+vty_transcript_test> single0
+ok argc=0