Move model overrides into own spec location

This fixes an auto-loading issue with spring which caused:

`MergeRequestWithoutMergeRequestDiff` causing superclass mismatch for
class

spec/factories/merge_requests.rb

@@ -290,9 +290,3 @@ FactoryBot.define do
     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

spec/support/models/merge_request_without_merge_request_diff.rb

+# frozen_string_literal: true
+
+class MergeRequestWithoutMergeRequestDiff < ::MergeRequest
+  self.inheritance_column = :_type_disabled
+
+  def ensure_merge_request_diff; end
+end