Frontend: Add feature test for Pipeline Editor tabs and linting

As a part of shifting left and optimizing E2E test suites https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/2542 , I think we can move test examples in E2E test pipeline_editor_tabs_spec.rb to feature spec.

• Verify all 4 tabs rendered on Pipeline Editor page (assume project already has a CI yml file): Edit, Visualize, Validate, Full configuration and Edit tab is selected by default
• Verify when CI yml has valid syntax:
  • Has validation "syntax is invalid"
  • Visualization tab has validation "syntax is invalid" and does not display any job
  • Gets simulation error when trying to simulate pipeline in Validate tab
  • Full configuration tab has source editor rendered and content is not empty
• Verify when CI yml has invalid syntax:
  • Has validation "Pipeline syntax is correct"
  • Visualization tab displays correct Jobs and Stages
  • Can simulate pipeline in Validate tab
  • Full configuration tab has source editor rendered and content is not empty