caddy-frontend: Unskip some tests

By using a late instance creation, it's possible to escape requirement of
the cluster being fully instantiated by default, so it's possible to test
various interesting corner cases.
12 jobs for failover-backend in 0 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:23:32

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:22:30

passed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

05:16:18

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

06:20:15

passed SlapOS.SlapRunner.ResilienceTest-Master

00:23:33

passed SlapOS.SlapRunner.ResilienceTest-Master

00:24:53

passed SlapOS.SlapRunner.ResilienceTest-Master

03:41:40

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:24:49

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:26:05

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:26:24

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:21:39

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

04:07:52