Use the first parser from 'treesit-parser-list' to fix tests.

Use the first parser from 'treesit-parser-list' to fix tests.

* lisp/treesit.el (treesit-parsers-at): Add treesit-primary-parser
only when it's non-nil.  When the result list is still empty,
add the first parser from 'treesit-parser-list'.
https://lists.gnu.org/archive/html/emacs-devel/2025-04/msg00627.html

* test/src/treesit-tests.el (treesit-node-supplemental)
(treesit-node-at, treesit-node-check)
(treesit-search-subtree-forward-1)
(treesit-search-subtree-backward-1): Wrap test body in 'with-temp-buffer'.

(cherry picked from commit ee46b6c4e67aa52b4ef683c5c16f64638e369cd3)