The removal check for the json tag of users.last_seen showed that the
subtest read the user API payloads only, which are built field by field.
It now marshals the model as well, so a tag that would expose an admin
column fails it.
- one subtest per threat id T-12.1-18 to T-12.1-25, 27 to 31, 34, 38 and 39
- T-12.1-28 revisits T-12-18: the groups field is the one writer of users_groups
- T-12.1-38 and T-12.1-39 cover the takeover guard for privileged-group members