Re-add recently-removed tests for ltree and intarray

Hi all,

Some of you may have noticed that some regression tests have been
removed due to some noise in the buildfarm, as of commit 906ea101d0d5.

We did not have time to do something for this release, unfortunately.
It is possible to reproduce the incompatibility by setting
max_stack_depth to a low value, where the first new query of ltree and
intarray would fail, when written in their original shape.

Tom had the idea to switch these two unstable tests to use a balanced
binary tree instead, so as they don't eat the stack still are able to
cover the recent fixes pushed into the tree.

And this investigation has led me to the attached, to-be-backpatched
down to v14. Even under a low max_stack_depth, these new tests are
stable. I could not see an issue for the two tests added at the
bottom of ltree.

Opinions or comments?

Thanks,
--
Michael