Hi,

I can confirm that `ocaml-mdx` is currently not building for me 
locally.

It seems to be that something in the dune 2.8 format changed and 
was breaking tests. This was fixed in ocaml-mdx 
(https://github.com/realworldocaml/mdx/pull/314) which is in 
ocaml-mdx 0.8. Simply updating ocaml-mdx to 0.9 (the latest) makes 
ocaml-mdx build again. Patch for that is attached.