caddy-frontend: Fix master partition find in the test

The real predicate to find the master partition is rejected slave publisher
configuration.
5 jobs for feat/debian11 in 0 seconds
latest
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:15:52

passed SlapOS.Eggs.UnitTest-Master.Python3

00:10:49

passed SlapOS.SlapRunner.ResilienceTest-Master

00:22:51

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

08:25:35

passed SlapOS.Eggs.UnitTest-Master.Python3

00:30:11