Extract ReplicableModel tests against Dummy

instead of writing these tests inside of replicator_spec.rb.

ee/spec/lib/gitlab/geo/replicable_model_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+# Also see ee/spec/support/shared_examples/models/concerns/replicable_model_shared_examples.rb:
+#
+# - Place tests here in replicable_model_spec.rb if you want to run them once,
+#   against a DummyModel.
+# - Place tests in replicable_model_shared_examples.rb if you want them to be
+#   run against every real Model.
+RSpec.describe Gitlab::Geo::ReplicableModel do
+  include ::EE::GeoHelpers
+
+  let_it_be(:primary_node) { create(:geo_node, :primary) }
+  let_it_be(:secondary_node) { create(:geo_node) }
+
+  before(:all) do
+    create_dummy_model_table
+  end
+
+  after(:all) do
+    drop_dummy_model_table
+  end
+
+  before do
+    stub_dummy_replicator_class
+    stub_dummy_model_class
+  end
+
+  subject { DummyModel.new }
+
+  it_behaves_like 'a replicable model' do
+    let(:model_record) { subject }
+  end
+
+  describe '#replicator' do
+    it 'adds replicator method to the model' do
+      expect(subject).to respond_to(:replicator)
+    end
+
+    it 'instantiates a replicator into the model' do
+      expect(subject.replicator).to be_a(Geo::DummyReplicator)
+    end
+  end
+end

ee/spec/support/helpers/ee/geo_helpers.rb

@@ -55,5 +55,67 @@ module EE
       # will appear as though the tracking DB were not available
       allow(::Gitlab::Geo).to receive(:geo_database_configured?).and_call_original
     end
+
+    def stub_dummy_replicator_class
+      stub_const('Geo::DummyReplicator', Class.new(::Gitlab::Geo::Replicator))
+
+      Geo::DummyReplicator.class_eval do
+        event :test
+        event :another_test
+
+        def self.model
+          ::DummyModel
+        end
+
+        def handle_after_create_commit
+          true
+        end
+
+        protected
+
+        def consume_event_test(user:, other:)
+          true
+        end
+      end
+    end
+
+    def stub_dummy_model_class
+      stub_const('DummyModel', Class.new(ApplicationRecord))
+
+      DummyModel.class_eval do
+        include ::Gitlab::Geo::ReplicableModel
+
+        with_replicator Geo::DummyReplicator
+
+        def self.replicables_for_geo_node
+          self.all
+        end
+      end
+
+      DummyModel.reset_column_information
+    end
+
+    # Example:
+    #
+    # before(:all) do
+    #   create_dummy_model_table
+    # end
+    #
+    # after(:all) do
+    #   drop_dummy_model_table
+    # end
+    def create_dummy_model_table
+      ActiveRecord::Schema.define do
+        create_table :dummy_models, force: true do |t|
+          t.binary :verification_checksum
+        end
+      end
+    end
+
+    def drop_dummy_model_table
+      ActiveRecord::Schema.define do
+        drop_table :dummy_models, force: true
+      end
+    end
   end
 end

ee/spec/support/shared_examples/models/concerns/replicable_model_shared_examples.rb

@@ -6,6 +6,13 @@
 #
 # We do not use `described_class` here, so we can include this in replicator
 # strategy shared examples instead of in *every* model spec.
+#
+# Also see ee/spec/lib/gitlab/geo/replicable_model_spec.rb:
+#
+# - Place tests in replicable_model_spec.rb if you want to run them once,
+#   against a DummyModel.
+# - Place tests here in replicable_model_shared_examples.rb if you want them to
+#   be run against every real Model.
 RSpec.shared_examples 'a replicable model' do
   include EE::GeoHelpers