Update the dvla response data to 0 page count

- as the response is fake, the notifications billable_unit is left at 0, the fake dvla response should also be 0. Otherwise there will be confusing logs reporting mismatched page count and billable units which are just research ones.

tests/app/celery/test_research_mode_tasks.py

@@ -124,7 +124,7 @@ def test_create_fake_letter_response_file_uploads_response_file_s3(
         create_fake_letter_response_file('random-ref')
 
         mock_s3upload.assert_called_once_with(
-            filedata='random-ref|Sent|1|Sorted',
+            filedata='random-ref|Sent|0|Sorted',
             region=current_app.config['AWS_REGION'],
             bucket_name=current_app.config['DVLA_RESPONSE_BUCKET_NAME'],
             file_location=filename