testing/ocaml*: some ppx updates and new aports

ocaml-bisect_ppx was holding back the upgrade of ocaml-ppxlib to 0.26.0. After finding a compatibility patch for bisect_ppx in upstream's Github pull requests, ppxlib can now finally be upgraded.

This should be ready to merge, and it doesn't involve dune-configurator, so it isn't necessary to wait for !34574 (merged) .

Edit: Oops, I see this MR got assigned to omni. Sorry, omni. I used your patchset from !25782 (closed) as a starting point, and back then you added yourself as maintainer for a few OCaml aports (ocaml-ounit and ocaml-yojson, as far as I can tell).

ocaml-ounit is the one involved in this MR, I enabled the oUnit "transition package", so hopefully those packages that go looking for ounit2 under its old name will now be able to run their tests.

I've been meaning to tell you that those changes have been merged, but never found the time. I hope you don't mind that you got added as maintainer for those 2 aports as a result of a previous MR of mine.