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/aws/s3.py

@@ -4,6 +4,7 @@ from flask import current_app
 
 import pytz
 from boto3 import client, resource
+import botocore
 
 FILE_LOCATION_STRUCTURE = 'service-{}-notify/{}.csv'
 
@@ -18,6 +19,17 @@ def get_s3_object(bucket_name, file_location):
     return s3.Object(bucket_name, file_location)
 
 
+def file_exists(bucket_name, file_location):
+    try:
+        # try and access metadata of object
+        get_s3_object(bucket_name, file_location).metadata
+        return True
+    except botocore.exceptions.ClientError as e:
+        if e.response['ResponseMetadata']['HTTPStatusCode'] == 404:
+            return False
+        raise
+
+
 def get_job_location(service_id, job_id):
     return (
         current_app.config['CSV_UPLOAD_BUCKET_NAME'],