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

changed milestone to %8.15

marked the task Added for this feature/bug as completed

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

marked the task Conform by the style guides as completed

marked the task Conform by the style guides as incomplete

marked the task Conform by the style 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

mentioned in issue #2772 (closed)

added 1 commit

• e3e6d174 - Rename Routable.where_paths_in to Routable.where_full_path_in

Compare with previous version

mentioned in merge request !7976 (closed)

added 36 commits

• e3e6d174...3f768f9a - 33 commits from branch master
• b552a4eb - Validate presence of route by Routable concern
• d7010fdc - Create nested group factory
• 3e0b8e00 - Rename Routable.where_paths_in to Routable.where_full_path_in

Compare with previous version

marked the task All builds are passing as completed

@DouweM please review

assigned to @DouweM

@smcgivern I see @DouweM on vacation. Can you please review?

assigned to @smcgivern

merged

mentioned in commit bd219711

LGTM!

I think we can use here build instead create

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8068