The code in #11616 has been working for a while (ever since 8.0.1),
so let's add a regression test for it to put the nail in the coffin.

Test Plan: make test TEST=T11616

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #11616

Differential Revision: https://phabricator.haskell.org/D3531