create fake letter response files with variables timestamps

when a test letter is created on dev or preview, we upload a file to
the dvla ftp response bucket, to test that our integration with s3
works. s3 triggers an sns notification, which we pick up, and then we
download the file and mark the letters it mentions as delivered.

However, if two tests run at the same time, they'll create the same
file on s3. One will just overwrite the next, and the first letter will
never move into delivered - this was causing functional tests to
intermittently fail.

This commit makes the test letter task check if the file exists - if it
does, it moves back one second and tries again. It tries this thirty
times before giving up.

app/celery/research_mode_tasks.py

@@ -1,4 +1,4 @@
-from datetime import datetime
+from datetime import datetime, timedelta
 import json
 
 from flask import current_app
@@ -7,6 +7,7 @@ from requests import request, RequestException, HTTPError
 from notifications_utils.s3 import s3upload
 
 from app import notify_celery
+from app.aws.s3 import file_exists
 from app.models import SMS_TYPE
 from app.config import QueueNames
 from app.celery.process_ses_receipts_tasks import process_ses_results
@@ -123,7 +124,18 @@ def firetext_callback(notification_id, to):
 def create_fake_letter_response_file(self, reference):
     now = datetime.utcnow()
     dvla_response_data = '{}|Sent|0|Sorted'.format(reference)
-    upload_file_name = 'NOTIFY-{}-RSP.TXT'.format(now.strftime('%Y%m%d%H%M%S'))
+
+    # try and find a filename that hasn't been taken yet - going back in time 60 seconds
+    for i in range(30):
+        upload_file_name = 'NOTIFY-{}-RSP.TXT'.format((now - timedelta(seconds=i)).strftime('%Y%m%d%H%M%S'))
+        if not file_exists(current_app.config['DVLA_RESPONSE_BUCKET_NAME'], upload_file_name):
+            break
+    else:
+        raise ValueError(
+            'cant create fake letter response file for {} - too many files for that time already exist on s3'.format(
+                reference
+            )
+        )
 
     s3upload(
         filedata=dvla_response_data,