Fix assign code owners spec

What does this MR do?

Fixes ee_assign_code_owners_spec.rb

There is a new setting checkbox under Settings > General > Merge request approvals called "Require approval from code owners" that needs to be enabled for the purpose of this test.

Closes: gitlab-org/quality/staging#39 (closed)

CE Port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30743

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content