Add feature examples specing maintainers creating subgroups

Both with subgroup_creation_level set to owners and to maintainers.
Also fixed the naming of some other examples in the same spec as they
were contradicting what they were actually performing in the test.
These examples were probably copy/pasted, and not renamed.

spec/features/groups/show_spec.rb

@@ -86,7 +86,7 @@ describe 'Group show page' do
           visit path
         end
 
-        it 'allows creating subgroups' do
+        it 'does not allow creating subgroups' do
           expect(page).not_to have_selector("li[data-text='New subgroup']", visible: false)
         end
       end
@@ -103,18 +103,32 @@ describe 'Group show page' do
           visit path
         end
 
+        context 'when subgroup_creation_level is set to maintainer' do
+          let(:group) { create(:group, subgroup_creation_level: ::Gitlab::Access::MAINTAINER_SUBGROUP_ACCESS) }
+          
           it 'allows creating subgroups' do
+            visit path
             expect(page).to have_css("li[data-text='New subgroup']", visible: false)
           end
         end
 
+        context 'when subgroup_creation_level is set to owners' do
+          let(:group) { create(:group, subgroup_creation_level: ::Gitlab::Access::OWNER_SUBGROUP_ACCESS) }
+
+          it 'does not allow creating subgroups' do
+            visit path
+            expect(page).not_to have_css("li[data-text='New subgroup']", visible: false)
+          end
+        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
+        it 'does not allow creating subgroups' do
           expect(page).not_to have_selector("li[data-text='New subgroup']", visible: false)
         end
       end