Users & Groups feature evolution

There is too many mis-understanding on this feature.

We should add a new main category called "Testing data", and add the Users & Groups sub-feature (there will also be a "document data" feature leveraging Thierry's new factory). We should explain on the description: Users & Groups testing data enables you to set users and groups for quickly playing your workflows, notifications, etc... Note that when you activate this feature, it override other authentication configuration (such as LDAP one) that would be on the deployment. You should be careful with it.