Add examples specing the setting to choose who can create subgroups

This setting is at the group level only. The default is specified to
be maintainers and owners.

**Specs only**, all failing.

spec/factories/groups.rb

@@ -5,6 +5,7 @@ FactoryBot.define do
     type 'Group'
     owner nil
     project_creation_level ::Gitlab::Access::MAINTAINER_PROJECT_ACCESS
+    subgroup_creation_level ::Gitlab::Access::OWNER_SUBGROUP_ACCESS
     after(:create) do |group|
       if group.owner

spec/support/shared_contexts/policies/group_policy_shared_context.rb

@@ -19,15 +19,10 @@ RSpec.shared_context 'GroupPolicy context' do
   let(:reporter_permissions) { [:admin_label] }
   let(:developer_permissions) { [:admin_milestone] }
   let(:maintainer_permissions) do
-    [
-      :create_projects,
-      :read_cluster,
-      :create_cluster,
-      :update_cluster,
-      :admin_cluster,
-      :add_cluster,
-      (Gitlab::Database.postgresql? ? :create_subgroup : nil)
-    ].compact
+    %i[
+      create_projects
+      read_cluster create_cluster update_cluster admin_cluster add_cluster
+    ]
   end
   let(:owner_permissions) do
     [
@@ -35,7 +30,8 @@ RSpec.shared_context 'GroupPolicy context' do
       :admin_namespace,
       :admin_group_member,
       :change_visibility_level,
-      :set_note_created_at
+      :set_note_created_at,
+      (Gitlab::Database.postgresql? ? :create_subgroup : nil)
     ].compact
   end