Tag a spec as :nested_groups since it fails on MySQL

This was broken by
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12322.

Signed-off-by: Rémy Coutable <remy@rymai.me>

spec/helpers/groups_helper_spec.rb

@@ -84,7 +84,7 @@ describe GroupsHelper do
     end
   end
 
-  describe 'group_title' do
+  describe 'group_title', :nested_groups do
     let(:group) { create(:group) }
     let(:nested_group) { create(:group, parent: group) }
     let(:deep_nested_group) { create(:group, parent: nested_group) }