vty_test: add artificial node levels for better testing

In vty_test, add three levels of parent nodes (level1, level2, level3) with
each having a leaf child (child1, child2, child3).

Use these to enhance the vty_test cfg files and test more diverse situations.

The current VTY code expects a go_parent_cb() to be present, otherwise it will
bump right back to the CONFIG_NODE, which will not work with more than one node
level below the CONFIG_NODE. Hence provide a minimal go_parent_cb().

Change-Id: Ib9bcf58b655fbd85e196f363fb7d8305d7dfc997
diff --git a/tests/vty/ok_ignore_comment.cfg b/tests/vty/ok_ignore_comment.cfg
index 5813fc7..1d6d87e 100644
--- a/tests/vty/ok_ignore_comment.cfg
+++ b/tests/vty/ok_ignore_comment.cfg
@@ -1,7 +1,8 @@
-line vty
+level1
   ! comment
-  no login
+  child1
 ! comment
-  no login
+  level2
      ! comment
-log stderr
+    child2
+level1