SecurityTestCase: fix username / user_id confusion

Theses methods have always been using user_id, but where written at
a time where there was not such disctinction
6 jobs for master in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:35:32

failed ERP5.PerformanceTest-Master

00:26:11

failed ERP5.UnitTest-Master

01:26:17

failed ERP5.UnitTest-Master.Medusa

01:28:26

passed SlapOS.Eggs.UnitTest-Master.Python2

00:17:13

passed SlapOS.Eggs.UnitTest-Master.Python3

00:12:51