Commit 18fbb182e8 changed the error
message for some conflicting GObject pspec flags, but the tests were not
updated to follow it. This wasn’t noticed because these tests are only
run with `-m thorough` as they’re a bit slow, so they only run weekly.
But we haven’t been noticing weekly CI run failures because the GitLab
issue-bot to report those failures has been broken for a long time. 😩
Signed-off-by: Philip Withnall <pwithnall@gnome.org>