make test dvla response file timestamps in a random order

since there'll be a bunch of threads running functional test tasks at
the same time, there's no point always trying to start from the same
second and then stepping back to the same one-second-back file each
time. Also, this leads us to an increased risk of race conditions.

This change takes the same thirty second range, but shuffles it. The
tests, since they're no longer deterministic, now use a new Matcher
object (w/ credit to alexey) to match any filename from within that
thirty second range

app/celery/research_mode_tasks.py

@@ -1,3 +1,4 @@
+import random
 from datetime import datetime, timedelta
 import json
 
@@ -125,8 +126,8 @@ def create_fake_letter_response_file(self, reference):
     now = datetime.utcnow()
     dvla_response_data = '{}|Sent|0|Sorted'.format(reference)
 
-    # try and find a filename that hasn't been taken yet - going back in time 60 seconds
-    for i in range(30):
+    # try and find a filename that hasn't been taken yet - from a random time within the last 30 seconds
+    for i in sorted(range(30), key=lambda _: random.random()):
         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