Resolve a couple of issue with qa/specs/features/ee/browser_ui/3_create/repository/code_owners_spec.rb

The following discussions from !122353 (merged) should be addressed:

• @mlapierre started a discussion:

  This is ok as a quick fix, but it could be a problem in the future if 'Show all' appears anywhere else on the page.

• @mlapierre started a discussion:

  This could be done via the API.

• It should use a Commit rather than ProjectPush to fabricate the project