create-normal-user: redirect stderr to stdout for adduser/useradd

create-normal-user is (now) called via check_exec which fails if there is output to stderr. However, if adduser or useradd don't actually fail, failing to run autopkgtest is bad, so let's redirect stderr to stdout (which is currently ignored the way check_exec is called).

Closes https://bugs.debian.org/1059725

An alternative which is equivalent is to redirect to /dev/null straight away (as check_exec does that already), but I like it a bit that at least manually running create-normal-user would actually show the output.

I tested with qemu on a stable testbed. Before the change it fails, after the change it looks good (still running, man what is that test time consuming).