E2E test: improve assertion for code owners spec

What does this MR do and why?

Resolves #512374 (closed)

The current assertion in this test searches the page for the username. In #512374 (closed) the code owners section wasn't present at all which made it difficult to debug.

This MR checks that the code owner section is explicitly checked first.

How to set up and validate locally

QA_GITLAB_URL=https://gdk.test:3443 bundle exec rspec qa/specs/features/ee/browser_ui/3_create/repository/code_owners_spec.rb