caddy-frontend/test: Request instance with slaves

This test just update slave dict of the class, so it is needed to request.
4 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SLAPOS-SR-TEST-MASTER

03:32:25

failed SLAPOS-SR-TEST-MASTER

03:39:51

failed SLAPOS-SR-TEST-MASTER

03:37:14

failed SLAPOS-SR-TEST-MASTER

03:52:28