Fix broken test T14547.

Phab:D4571 lags behind HEAD for too many commits. The commit of
Phab:4571 1f88f541 brought some
unintentional changes (not belong to [Phab:4571's Diff
16314](https://phabricator.haskell.org/differential/diff/16314/)) into
ghc-head, breaking T14557.

Let's fix that.

Test Plan: make test TEST="T14547"

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15222

Differential Revision: https://phabricator.haskell.org/D4778