Use subclass to override ensure_merge_request_diff

No longer use stubbing for `MergeRequest#ensure_merge_request_diff`, but
instead create a `MergeRequest` that overrides
`#ensure_merge_request_diff` with an empty implementation.

ee/spec/models/geo/merge_request_diff_registry_spec.rb

@@ -3,9 +3,7 @@
 require 'spec_helper'
 
 RSpec.describe Geo::MergeRequestDiffRegistry, :geo, type: :model do
-  let_it_be(:merge_request) { create(:merge_request) }
-  let_it_be(:merge_request_diff) { create(:merge_request_diff, merge_request: merge_request) }
-  let_it_be(:registry) { create(:geo_merge_request_diff_registry, merge_request_diff: merge_request_diff) }
+  let_it_be(:registry) { create(:geo_merge_request_diff_registry) }
 
   specify 'factory is valid' do
     expect(registry).to be_valid

spec/factories/merge_request_diffs.rb

@@ -2,13 +2,7 @@
 
 FactoryBot.define do
   factory :merge_request_diff do
-    merge_request do
-      build(:merge_request) do |merge_request|
-        # MergeRequest should not create a MergeRequestDiff in the callback
-        allow(merge_request).to receive(:ensure_merge_request_diff)
-      end
-    end
-
+    association :merge_request, factory: :merge_request_without_merge_request_diff
     state { :collected }
     commits_count { 1 }
 

spec/factories/merge_requests.rb

@@ -286,5 +286,13 @@ FactoryBot.define do
         merge_request.update!(labels: evaluator.labels)
       end
     end
+
+    factory :merge_request_without_merge_request_diff, class: 'MergeRequestWithoutMergeRequestDiff'
   end
 end
+
+class MergeRequestWithoutMergeRequestDiff < ::MergeRequest
+  self.inheritance_column = :_type_disabled
+
+  def ensure_merge_request_diff; end
+end