Minor improvements to nested groups code

What does this MR do?

1. Added nested group factory for tests
2. Validate presence of route by Routable concern
3. Rename Routable.where_paths_in to Routable.where_full_path_in based on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7121?view=inline#note_19746337

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

no

Why was this MR needed?

• Ensure namespace/project has a route.
• Nested group factory will be needed for future tests

Screenshots (if relevant)

no

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • 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