Fix: Geo::ModelMapper flakiness

What does this MR do and why?

Sometimes, when a spec (ee/spec/lib/gitlab/geo/replicator_spec.rb) stubs a replicator runs previously, the Geo::DummyReplicator will exist and be added to the Replicator subclasses used by the ModelMapper. However, the dummy model won't exist anymore - making model_class undefined.

This MR rescues the NameError and creates a dummy class instead removes the dummy replicator from the tested list, so that specs do not fail anymore due to this state leakage.

References

• gitlab-org/quality/engineering-productivity/master-broken-incidents#14684 (closed)
• #554463 (closed)

How to set up and validate locally

1. Checkout master
2. Run the spec with bundle exec rspec ee/spec/lib/gitlab/geo/replicator_spec.rb ee/spec/lib/gitlab/geo/model_mapper_spec.rb. It fails 😥
3. Now checkout this MR's branch
4. Run the spec with the same command. It passes 🎉

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.