Detect specs that would fail with subgroups

Idea extracted from https://gitlab.com/gitlab-org/gitlab-ce/issues/30790#note_28147104.

Steps:

1. Create a branch
2. Change the group's factory to always create a subgroup
3. Gather failing specs
4. Write specific specs for the subgroup use-case corresponding to the failing specs from previous step
5. Fix the implementation

/cc @DouweM @dzaporozhets