Would you consider the following patch? It makes autopkgtest work for me:
diff --git a/setup-commands/create-normal-user b/setup-commands/create-normal-user index 0d0e828..9d66694 100755 --- a/setup-commands/create-normal-user +++ b/setup-commands/create-normal-user @@ -21,7 +21,7 @@ create () { adduser \ --disabled-login \ --gecos 'Temporary autopkgtest user,,,' \ - "$user" 2>&1 + "$user" 2>&1 | grep -v 'send message failed: Operation not permitted' elif command -v useradd >/dev/null; then useradd \ --comment 'Temporary autopkgtest user' \