selftest: Make check_updates just work on cache by default

templates/selftest/commands.conf.erb

@@ -15,5 +15,8 @@ object CheckCommand "updates" {
       set_if = "$updates_security_only$"
     }
     "-t" = "$updates_timeout$"
+    "--yum-arguments" = "$updates_yum_arguments$"
   }
+
+  vars.updates_yum_arguments = "--cacheonly"
 }