Add development fixtures for nested groups

What does this MR do?

Adds development fixtures for nested groups

Are there points in the code the reviewer needs to double check?

no

Why was this MR needed?

For easy use and test of nested groups feature

Screenshot

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/2772

changed milestone to %9.0

marked the task Conform by the merge request performance guides as completed

marked the task Branch has no merge conflicts with master (if it does - rebase it please) as completed

marked the task Squashed related commits together as completed

marked the task Conform by the style guides as completed

@DouweM please review

assigned to @DouweM

marked the task All builds are passing as completed

marked the task All builds are passing as incomplete

merged