Use shared examples to increase experiment test coverage of BaseNamespaceCreateService

What does this MR do and why?

• Makes code DRYer by using shared_example for experiments
• Increases test coverage by testing:
  • Experiments in ee/spec/services/registrations/import_namespace_create_service_spec.rb via shared_example
  • That when group creation fails no experiments are tracked
  • For default_pinned_nav_items: That experiment is not tracked, when trial is not applied

References

Improve experiment rubocop when using abstract ... (#571739)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #571739