Reproduction case for #11824

[Leonidas-from-XIV]

This PR adds a test to show that portable lockfiles currently don't output the depexts correctly.

I looked into the lockfiles and the depexts are recorded correctly, it's just show depext which doesn't resolve them.

[gridbugs]

I understand why this happens. Dune currently only records depexts into lockfiles for platforms that it solves for rather than all the platforms listed in the opam file. I think for depexts it might make sense to record all the metadata from the opam file into the lockfile, since the choice of depexts is often more specific than dependencies. For example for the more obscure linux distributions it's probably fine to solve dependencies without considering os-distribution or os-version, however the choice of depexts usually requires considering these fields.

I'll explore an alternative way of recording depexts that preserves all the information about depexts in the opam file.

[Leonidas-from-XIV]

Yes, that explains very well why the repro test for the bug passes on my machine (Fedora) while it fails (aka, the behavior is correct) on CI (which presumably uses something Debian-like).