Add test for ids format in super admin list

Author: ernilambar

features/super-admin.feature

@@ -24,6 +24,12 @@ Feature: Manage super admins associated with a multisite instance
       superadmin
       """
 
+    When I run `wp super-admin list --format=ids`
+    Then STDOUT should be:
+      """
+      admin superadmin
+      """
+
     When I try `wp super-admin add superadmin`
     Then STDERR should be:
       """