testsuite: Add regression test for #23864

Simon says this was fixed by

commit 59202c80
Author: Sebastian Graf <sebastian.graf@kit.edu>
Date:   Fri Mar 31 17:35:22 2023 +0200

    CorePrep: Eliminate EmptyCase and unsafeEqualityProof in CoreToStg instead

    We eliminate EmptyCase by way of `coreToStg (Case e _ _ []) = coreToStg e` now.
    The main reason is that it plays far better in conjunction with eta expansion
    (as we aim to do for arguments in CorePrep, #23083), because we can discard
    any arguments, `(case e of {}) eta == case e of {}`, whereas in `(e |> co) eta`
    it's impossible to discard the argument.