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.cfg b/tests/vty/ok.cfg
index d5ef23e..f562348 100644
--- a/tests/vty/ok.cfg
+++ b/tests/vty/ok.cfg
@@ -1,3 +1,28 @@
-line vty
- no login
-log stderr
+level1 a
+ child1 a
+ level2 a
+  child2 a
+  level3 a
+   child3 a
+level1 b
+ child1 b
+ level2 b
+  child2 b
+level1 c
+ child1 c
+ level2 c
+  child2 c
+  level3 c
+   child3 c
+ level2 d
+  child2 d
+level1 e
+ child1 e
+ level2 e
+  child2 e
+ level2 f
+  child2 f
+level1 g
+ child1 g
+level1 h
+ child1 h