E2E test: use API for codeowner test

What does this MR do and why?

Resolves the remainder of #413915 (closed)

• Adds members to project via API (saves about 10-15s)
• Uses data-testid for codeowners toggle to be more specific

How to set up and validate locally

bundle exec rspec qa/specs/features/ee/browser_ui/3_create/repository/code_owners_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.