Replace admin sidebar feature spec with view spec

What does this MR do?

This replaces spec/features/admin/admin_active_tab_spec.rb with spec/views/layouts/nav/sidebar/_admin.html.haml_spec.rb, reducing the spec duration from 22 seconds to 12 seconds.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer