Spec: Group burndown chart considers all issues nesting down in all subgroups' projects

• Now that issues can take on milestones from ancestor groups (https://gitlab.com/gitlab-org/gitlab-ce/issues/36862), we should have the group burndown chart consider all issues nesting down in it's calculation.
• I.e. given a burndown chart for milestone m. All issues that have this milestone should be considered in the calculation, even if those issues are in further nested down subgroups.
• Per https://gitlab.com/gitlab-org/gitlab-ee/issues/6268#note_76876746, the requirement of this issue is to add a spec to verify the functionality.