Add failing feature spec detailing a maintainer creating a subgroup

- Change the two existing feature examples that create a subgroup to
  elucidate that the owner is creating the subgroup

- Nest two more specs inside the 'subgroup support' context detailing
  what happens when a maintainer attempts to add a subgroup (one with
  subgroup support, and one without)

spec/features/groups/show_spec.rb

@@ -56,11 +56,17 @@ describe 'Group show page' do
   end
 
   context 'subgroup support' do
-    let(:user) { create(:user) }
+    let(:owner) { create(:user) }
+    let(:maintainer) { create(:user) }
 
     before do
-      group.add_owner(user)
-      sign_in(user)
+      group.add_owner(owner)
+      group.add_maintainer(maintainer)
+    end
+
+    context 'for owners' do
+      before do
+        sign_in(owner)
       end
 
       context 'when subgroups are supported', :js, :nested_groups do
@@ -86,6 +92,35 @@ describe 'Group show page' do
       end
     end
 
+    context 'for maintainers' do
+      before do
+        sign_in(maintainer)
+      end
+
+      context 'when subgroups are supported', :js, :nested_groups do
+        before do
+          allow(Group).to receive(:supports_nested_objects?) { true }
+          visit path
+        end
+
+        it 'allows creating subgroups' do
+          expect(page).to have_css("li[data-text='New subgroup']", visible: false)
+        end
+      end
+
+      context 'when subgroups are not supported' do
+        before do
+          allow(Group).to receive(:supports_nested_objects?) { false }
+          visit path
+        end
+
+        it 'allows creating subgroups' do
+          expect(page).not_to have_selector("li[data-text='New subgroup']", visible: false)
+        end
+      end
+    end
+  end
+
   context 'group has a project with emoji in description', :js do
     let(:user) { create(:user) }
     let!(:project) { create(:project, description: ':smile:', namespace: group) }