Recover test case for T11547

commit 98c7749c has reverted commit 59d7ee53, including the test that
that file added. That test case is still valuable, so I am re-adding it.
I add it with it’s current (broken) behavior so that whoever fixes it
intentionally or accidentially will notice and then commit the actual
desired behavior (which is kinda unspecified, see
ghc/ghc#20455 (comment 382030))