Fix an issue when searching subtree backward (bug#67117)

Fix an issue when searching subtree backward (bug#67117)

* src/treesit.c (treesit_traverse_child_helper):
Do not call treesit_traverse_sibling_helper when the named node is
required and the last child is the named node.
Otherwise treesit_traverse_sibling_helper will move cursor to the
previous sibling and last node will be skipped.
* test/src/treesit-tests.el (treesit-search-subtree-forward-1):
(treesit-search-subtree-backward-1):
Add tests.

(cherry picked from commit 7b315e8a5c966f8d11a4f646db4e29b989b56ab1)