Add a validity testcase for uninhabited variants

[meithecatte]

Found this testcase in a random unmerged branch of mine. Looks like I wrote it as part of my work on rust-lang/rust#138961, and then forgot to submit it when plans wrt the scope of that PR have changed.

I have double checked by grepping for the error message and it looks like we still don't have any similar tests in miri.