Improve SaaS testing and linting of ee vs ce code

Plan

• Move metadata of :saas to foss in order to leverage it there - !112833 (merged)
• Make data-testid a helper for rspec tests - !129391 (merged)
• Enable :saas_registration metadata in tests to automatically include :saas metadata - !112861 (merged)
• Explore linting for code placement for foss extended into ee vs standalone
  • #378167 (comment 1214812437)

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The following discussion from !100466 (merged) should be addressed:

• @dstull started a discussion: (+5 comments)

  issue:

  If this is not a prepended(extended) class from foss, then it should live under ee/app/models/preloaders/user_member_roles_in_projects_preloader.rb instead(models/preloaders instead of models/ee/preloaders) and not need this module EE line see ee/app/models/preloaders/environments/protected_environment_preloader.rb for an example.

  side thought - we need to build better mouse trap in some way for this...some kindof linter as it should be programmatically possible to lint this.