Filter test messages from jobs on the dashboard

While test messages technically have a file and are a job, there’s not
much reason to ever revisit them. So all they end up doing is cluttering
the dashboard and making it harder to find the actual files you’ve
actually uploaded from your computer.

So this commit:

- abstracts the name of test messages into config
- filters out any files whose filename represents a test message
- adds some more thorough tests for the jobs on the dashboard

config.py

@@ -40,6 +40,7 @@ class Config(object):
     DESKPRO_DEPT_ID = os.environ['DESKPRO_DEPT_ID']
     DESKPRO_ASSIGNED_AGENT_TEAM_ID = os.environ['DESKPRO_ASSIGNED_AGENT_TEAM_ID']
     ACTIVITY_STATS_LIMIT_DAYS = 7
+    TEST_MESSAGE_FILENAME = 'Test message'
 
     EMAIL_DOMAIN_REGEXES = [
         "gov\.uk",