Fix a migration spec messing up the MergeRequestDiff DB schema

spec/lib/gitlab/background_migration/populate_merge_request_metrics_with_events_data_spec.rb

@@ -7,6 +7,10 @@ describe Gitlab::BackgroundMigration::PopulateMergeRequestMetricsWithEventsData,
       .to receive(:commits_count=).and_return(nil)
   end
 
+  after do
+    [MergeRequest, MergeRequestDiff].each(&:reset_column_information)
+  end
+
   describe '#perform' do
     let(:mr_with_event) { create(:merge_request) }
     let!(:merged_event) { create(:event, :merged, target: mr_with_event) }