software/caddy-frontend/test: use resource.Session as context manager

this prevents ResourceWarning because this way the sockets are closed
6 jobs for fix/caddy-test-resource-warning in 0 seconds
Status Job ID Name Coverage
  External
running Cloudooo.UnitTest-Master

19596:13:13

running SlapOS.Eggs.UnitTest-Master.Python3

19596:12:58

running SlapOS.SlapRunner.ResilienceTest-Master.ERP5

19596:13:03

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP-Python3

02:14:45

running SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

19596:13:10

running SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

19596:13:15