Add a CI step to test named -u

In light of #535 (closed), it seems worthwhile to add a CI step that runs as root and attempts to switch UID.

Gitlab CI does run at least one thing as root (ifconfig.sh) so this ought to be possible.

I can write the test, but I don't know how to make it work on the runner, so I'll assign the issue to @ondrej to begin with.