See !27746

- Fix rubocop disable comments
- Move `before` block to before tests
- Correct rubocop alerts due to new rules
- Use shared examples to clarify Go proxy spec
- Enable HTTP Basic authentication for Go proxy
  + Support both HTTP basic and normal token header/query var
  + Remove custom `find_project!` helper and use basic auth helpers
- Validate GoModuleVersion type attribute
- Implement testing factories
- Implement specs for untested new classes
- Add a Settings helper for Go URLs