ws: Fix user group detection in test-remotectlcertificate
ws: Fix user group detection in test-remotectlcertificate

`getgrnam (g_get_user_name ())` is not valid -- that function expects a
group name, not a user name. This only worked because on most system
the user's primary group has the same name as the user.

As our tests assume having a group anyway, convert the condition into an
assertion. They would not have succeeded anyway on a system where the
user does not have an existing group.

martinpitt/cockpit,garrett/cockpit,mvollmer/cockpit,garrett/cockpit,mvollmer/cockpit,martinpitt/cockpit,garrett/cockpit,mvollmer/cockpit,cockpit-project/cockpit,garrett/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,garrett/cockpit