test: Enable multi-host cockpit feature for TestMultiMachineVNC

This test does a direct remote host connection via URL change, which is
forbidden by default on newer OSes since [1]. Enable multi-host feature
for this test.

cockpit-project/cockpit@4cb9238994ba

test/check-machines-multi-host-consoles

@@ -42,6 +42,7 @@ class TestMultiMachineVNC(VirtualMachinesCase):
         self.machine2 = self.machines['machine2']
 
         self.setup_ssh_auth()
+        self.enable_multihost(self.machine1)
 
     def testBasic(self):
         b = self.browser