bug#7320: id and groups may lie

[Jim Meyering]

Marc W. Mengel wrote:
> The other test case is to make a copy of "id" and make it
> setuid to some user (i.e. mysql) and run it; it will show
> itself as having mysql's primary group, even though it doesn't.

Oh!  Yes, that will work.  Thanks.
With that, I'll add a test like this:

New:
    $ sudo src/setuidgid -g 3 root src/id -G
    3

Old:
    $ sudo src/setuidgid -g 3 root id -G
    3 0