Add (failing) test coverage for !23

This demonstrates the behaviour we want from !23.

Signed-off-by: Simon McVittie <smcv@collabora.com>
3 jobs for master in 8 minutes and 21 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
passed #20722
build:bionic

00:08:01

passed #20721
build:buster

00:07:27

passed #20720
build:stretch

00:08:21