tests: cover gpasswd as a group administrator

[pierre-warnier]

Follow-up to #282, as promised there.

The reason gpasswd is setuid is that someone who is not root, but is named
an administrator of one group, manages that group's membership. Nothing in the
suite exercised a non-root caller of it — and that is the only place the
omission shows. The multicall binary drops privilege for every applet not on
its setuid list, so leaving gpasswd off that list produces a permission error
there and nowhere else. It was off the list when the tool arrived.

Twelve assertions: an administrator adds a member without being root and the
change lands in both /etc/group and /etc/gshadow; a plain member may not
change the membership, may not appoint administrators, and may not use
--root; and the administrator list is unchanged after all three refusals.

The test bites

Taking gpasswd back off SETUID_APPLETS and rebuilding:

=== admin adds a member, gpasswd OFF the setuid list ===
gpasswd: Permission denied
exit=1
t3:x:1005:                      ← group unchanged

Worth recording that my first attempt at this check appeared to show the fix
was unnecessary: I had suppressed the build output, cargo had not rebuilt, and
a stale binary went in. The lesson is the usual one — do not hide the output of
the step whose result you are about to reason from.

Deployment suite        235 passed, 0 failed
make check              exit 0