**Problem:** `is_ancestor()` uses a slow, bottom-up parent lookup which
has performance pitfalls detailed in #28512.
**Solution:** Take `is_ancestor()` from $O(n^2)$ to $O(n)$ by
incorporating the use of the `child_containing_descendant()` function
Problem:
Tests have lots of exec_lua calls which input blocks of code
provided as unformatted strings.
Solution:
Teach exec_lua how to handle functions.
Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.
Closes https://github.com/neovim/neovim/issues/27004.
