this is for the field p_person.p_family_id_i
openpetraorg.sourceforge.net/dbd ... up=partner
A family consists of several persons, all with the same p_family_key_n.
The family id is unique among the members of that family. usually the person with the main job at the organisation, has id 0.
the children have ID 2, 3, 4 etc.
if a child is moved out of a family, or the parents family id is switched, this test should check if the family id is still consistent across all members of that family.