test: use `useradd` instead of `adduser`

test: use `useradd` instead of `adduser`

`adduser` is in certain cases a standalone package which provides a
better user experience. In other cases it's just a symlink to `useradd`.
And some distributions don't have `adduser` at all, like Arch Linux.
Let's use the `useradd` binary instead, which should provide the same
functionality everywhere.