Add failing unit test specifying a maintainer creating a subgroup

spec/services/groups/create_service_spec.rb

+# coding: utf-8
 # frozen_string_literal: true
 
 require 'spec_helper'
@@ -87,6 +88,14 @@ describe Groups::CreateService, '#execute' do
 
         it { is_expected.to be_persisted }
       end
+
+      context 'as maintainer' do
+        before do
+          group.add_maintainer(user)
+        end
+
+        it { is_expected.to be_persisted }
+      end
     end
   end
 

spec/support/shared_contexts/policies/group_policy_shared_context.rb

@@ -23,6 +23,15 @@ RSpec.shared_context 'GroupPolicy context' do
       create_projects
       read_cluster create_cluster update_cluster admin_cluster add_cluster
     ]
+    [
+      :create_projects,
+      :read_cluster,
+      :create_cluster,
+      :update_cluster,
+      :admin_cluster,
+      :add_cluster,
+      (Gitlab::Database.postgresql? ? :create_subgroup : nil)
+    ].compact
   end
   let(:owner_permissions) do
     [
@@ -30,8 +39,7 @@ RSpec.shared_context 'GroupPolicy context' do
       :admin_namespace,
       :admin_group_member,
       :change_visibility_level,
-      :set_note_created_at,
-      (Gitlab::Database.postgresql? ? :create_subgroup : nil)
+      :set_note_created_at
     ].compact
   end