Feature wish: provide tooling for removing/disabling mdx checks when running under opam tests

[raphael-proust]

The default mdx stanza causes hard fails (non-zero return code). This is god for local development and for a project's own CI (it forces you to update the doc!).

However, it is not always appropriate for running in opam install --with-test because regressions in mdx output (even just whistespace formatting) causes failures. See ocaml/opam-repository#25459 for revdeps issues that have happened in the past.

It would be useful to make the mdx stanza non-fatal in release profile, or something similar.