Add e2e test: auto-assign code owners

What does this MR do?

Add a new end-to-end test that checks that a code owner is automatically assign as an approver based on the current CODEOWNERS file in master.

CE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22939

What are the relevant issue numbers?

• Test plan: https://gitlab.com/gitlab-org/gitlab-ee/issues/7959
• Feature: https://gitlab.com/gitlab-org/gitlab-ee/issues/1012

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• ~~Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?