test: Gracefully handle running within user namespace with single user

test: Gracefully handle running within user namespace with single user

Unprivileged users often make themselves root by unsharing a user namespace
and then mapping their current user to root which does not require privileges.
Let's make sure our tests don't fail in such an environment by adding checks
where required to see if we're not running in a user namespace with only a
single user.

(cherry picked from commit ef31767ed7e21672a50b77e7b3935948aaba114c)