This was added in commits 447d9bbc and fb6328b9.
With it, the validation succeeds. Still I am actually not sure that is
proper appdata format. Also we have issues of metadata not shown for our
flatpak package, and I am wondering if this could be the issue.
So let's get rid of this hack.

Also temporarily commenting out the appdata unit test because it fails
without, because of a bug in appstream-util. This bug has been fixed
since, but there are no releases containing this fix yet.
See: https://github.com/hughsie/appstream-glib/issues/234
We'll reactivate the tests later.