Fix `check_belongs_to_group` for non-unique uids
Fix `check_belongs_to_group` for non-unique uids

`id` outputs the first uid that matches when presented with potentially multiple users with the same uid. This means that if 2 users with the same uid are present, `check_belongs_to_group` may fail even if the user is part of the group.
mizzy/specinfra,serverspec/specinfra